Antiviral prodrugs of tenofovir

ABSTRACT

Compounds of Formula I: and pharmaceutically acceptable salts and co-crystals thereof are useful for the inhibition of HIV reverse transcriptase. The compounds may also be useful for the prophylaxis or treatment of infection by HIV and in the prophylaxis, delay in the onset or progression, and treatment of AIDS. The compounds and their salts can be employed as ingredients in pharmaceutical compositions, optionally in combination with other antiviral agents, immunomodulators, antibiotics or vaccines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 filing from InternationalApplication No. PCT/US2017/047892 filed August 22, 2017, which claimspriority to U.S. Provisional Application No. 62/379,455, filed Aug. 25,2016.

The aforementioned application to which this application claims priorityis herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The retrovirus designated human immunodeficiency virus (HIV),particularly the strains known as HIV type-1 (HIV-1) and type-2 (HIV-2),have been etiologically linked to the immunosuppressive disease known asacquired immunodeficiency syndrome (AIDS). HIV seropositive individualsare initially asymptomatic but typically develop AIDS related complex(ARC) followed by AIDS. Affected individuals exhibit severeimmunosuppression which makes them highly susceptible to debilitatingand ultimately fatal opportunistic infections. Replication of HIV by ahost cell requires integration of the viral genome into the host cell'sDNA. Since HW is a retrovirus, the HIV replication cycle requirestranscription of the viral RNA genome into DNA via an enzyme known asreverse transcriptase (RT).

Reverse transcriptase has three known enzymatic functions: The enzymeacts as an RNA-dependent DNA polymerase, as a ribonuclease, and as aDNA-dependent DNA polymerase. In its role as an RNA-dependent DNApolymerase, RT transcribes a single-stranded DNA copy of the viral RNAAs a ribonuclease, RT destroys the original viral RNA and frees the DNAjust produced from the original RNA. During the viral RNA-dependentpolymerization process, RT's ribonuclease activity is required forremoving RNA and leaving the polypurine tract preserved for initiationof DNA-dependent polymerization. As a DNA-dependent DNA polymerase, RTmakes a second, complementary DNA strand using the first DNA strand as atemplate. The two strands form double-stranded DNA, which is integratedinto the host cell's genome by HIV integrase.

It is known that compounds that inhibit enzymatic functions of HIV RTwill inhibit HIV replication in infected cells. These compounds areuseful in the treatment of HIV infection in humans. There are twoclasses of RT inhibitors: one is non-nucleoside active site competitiveRT inhibitors (NNRTIs), such as efavirenz (EFV), nevirapine (NVP),etravirine (ETR), and rilpivirine (RPV), and the other is active site RTinhibitors which include nucleoside reverse transcriptase inhibitors(NsRTIs) and nucleotide reverse transcriptase inhibitors (NtRTIs),collectively referred to as NRTIs. Examples of NsRTI's include3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxyinosine (ddI),2′,3′-dideoxycytidine (ddC), 2′,3′-didehydro-2′,3′-dideoxythymidine(d4T), 2′,3′-dideoxy-3′-thiacytidine (3TC), abacavir, and emtricitabine.Examples of NtRTIs include tenofovir (TFV, also known as PMPA,9-(2-phosphonyl-methoxypropyl)adenine), tenofovir disoproxil fumarate(VIREAD®, U.S. Pat. Nos. 5,977,089, 5,935,946) and tenofovir alafenamidefumarate (U.S. Pat. Nos. 7,390,791, 8,754,065).

TFV belongs to a class of HIV anti-retroviral (ARV) agents known asnucleotide analog reverse transcriptase inhibitors (NRTIs). Tenofovir isa monophosphonate:

After being taken up by cells, TFV is first converted totenofovir-monophosphate (TFV-MP) by adenosine monophosphate kinase andthen to the active antiviral tenofovir-diphosphate (TFV-DP) by5′-nucleoside diphosphate kinase.

TFV-DP inhibits HIV DNA synthesis by competing with the naturalsubstrate, deoxyadenosine triphosphate, for incorporation into thecomplementary DNA strand by HIV reverse transcriptase; followingincorporation, TFV acts as a chain terminator due to lack of a3′-hydroxyl group that is required for addition of the next nucleotide.TFV has poor cellular permeability and thus has limited bioavailability.Tenofovir disoproxil fumarate (TDF) is approved for treating HIVinfection and is marketed by Gilead under the trade name VIREAD™. Thedisoproxil prodrug improves cell permeability and absorption after oraldosing, with the pro-moiety being cleaved rapidly after absorption toyield the parent TFV. As a result, the circulating level of TFV is muchhigher than that of TDF. Tenofovir alafenamide fumarate (TAF) iscurrently approved by the USFDA as an active ingredient in combinationwith additional ARVs for treating HIV infection in the pharmaceuticalproducts GENVOYA®, ODEFSEY® and DESCOVY®.

While each of the foregoing drugs is effective in treating HIV infectionand AIDS, there remains a need to develop additional HIV antiviral drugsincluding additional RT inhibitors. A particular problem is thedevelopment of mutant HIV strains that are resistant to the knowninhibitors. The use of RT inhibitors to treat AIDS often leads toviruses that are less sensitive to the inhibitors. This resistance istypically the result of mutations that occur in the reversetranscriptase segment of the pol gene. The continued use of antiviralcompounds to prevent HIV infection will inevitably result in theemergence of new resistant strains of HIV. Accordingly, there is aparticular need for new RT inhibitors that are effective against mutantHIV strains.

SUMMARY OF THE INVENTION

The present invention is directed lactate and glycolate prodrugs oftenofovir and their use in the inhibition of nucleotide reversetranscriptase. In addition to the use of said compounds in theinhibition of HIV reverse transcriptase, the invention is also directedto the use of said compounds for prophylaxis of infection by HIV, thetreatment of infection by HIV, and the prophylaxis, treatment, and/ordelay in the onset or progression of AIDS and/or ARC.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a compound of structural Formula I:

or a pharmaceutically acceptable salt thereof, wherein:R¹ and R² are each independently selected from (a) H, (b) —C₁₋₄alkyl,(c) —C₁₋₄alkyl substituted with —OH, —SH, —SCH₃, —NH₂ or —NH—C(═NH)—NH₂,(d) —CH₂-phenyl, (e) —CH₂-phenol, (f) —(CH₂)₁₋₂—COOH, (g)—(CH₂)₁₋₂—CONH₂, (h) —CH₂-1H-indole, (i) —CH₂-imidazole, (j) aryl (forexample but not limited to phenyl or naphthyl) or (k) heteroaryl (forexample but not limited to pyridine);R³ is selected from:(a) —C₁₋₁₀alkyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(7a), —SH, —NR^(9a)R^(10a), —C₃₋₆cycloalkyl or spiro—C₃₋₆cycloalkyl,(b) —CH₂-phenyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl,(c) —C₃₋₈cycloalkyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl,(d) aryl unsubstituted or substituted with one to three substituentsindependently selected from fluoro, chloro, bromo, —OR^(8a), —SH,—NR^(9a)R^(10a) or —C₁₋₃alkyl,(e) —C₁₋₅alkyl—X—C₁₋₅alkyl wherein X is O, S or NH,(f) heteroaryl unsubstituted or substituted with one to threesubstituents independently selected from fluoro chloro, bromo, —OR^(8a),—SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, or(g) a heterocyclic ring unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl;R⁴ is selected from:(a) —C₁₋₁₀alkyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(7b), —SH, —NR^(9b)R^(10b), —C₃₋₆cycloalkyl or spiro—C₃₋₆cycloalkyl,(b) —CH₂-phenyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl,(c) —C₃₋₈cycloalkyl unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl,(d) aryl unsubstituted or substituted with one to three substituentsindependently selected from fluoro, chloro, bromo, —OR^(8b), —SH,—NR^(9b)R^(10b) or —C₁₋₃alkyl,(e) —C₁₋₅alkyl—X—C₁₋₅alkyl wherein X is O, S or NH,(f) heteroaryl unsubstituted or substituted with one to threesubstituents independently selected from fluoro chloro, bromo, —OR^(8b),—SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, or(g) a heterocyclic ring unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl;R⁵ and R⁶ are each independently selected from (a) —C₁₋₄alkyl, (b)—C₁₋₄alkyl substituted with —OH, —SH, —SCH₃, —NH₂, —NH—C(═NH)—NH₂, (c)—CH₂-phenyl, (d) —CH₂-phenol, (e) —(CH₂)₁₋₂—COOH, (f) —(CH₂)₁₋₂—CONH₂,(g) —CH₂-1H-indole, (h) —CH₂-imidazole, (i) aryl (for example but notlimited to phenyl or naphthyl) or (j) heteroaryl (for example but notlimited to pyridine);or R⁵ and R⁶ are joined together with the carbon to which they are bothattached to form —C₃₋₆cycloalkyl or a 4 to 6-membered heterocyclic ring;R^(7a) and R^(7b) are each independently selected from —H or—C₃₋₆cycloalkyl;R^(8a) and R^(8b) are each independently selected from —H, —C₁₋₃alkyl or—C₃₋₆cycloalkyl;R^(9a) and R^(10a) are each independently selected from —H, —C₁₋₃alkylor —C₃₋₆cycloalkyl; andR^(9b) and R^(10b) are each independently selected from —H, —C₁₋₃alkylor —C₃₋₆cycloalkyl.

In Embodiment 1 of this invention are compounds of Formula I havingstructural Formula Ia:

or a pharmaceutically acceptable salt thereof, wherein the variables areas defined in Formula I.

In Embodiment 2 of this invention are compounds of Formula I, Formula Iaor pharmaceutically acceptable salts thereof, wherein R¹ is —H or—C₁₋₄alkyl. In a class thereof, R¹ is —H or —CH₃.

In Embodiment 3 of this invention are compounds of Formula I, Formula Iaor Embodiment 2, or any class thereof, or pharmaceutically acceptablesalts of the foregoing, wherein R² is —H, —C₁₋₄alkyl or —CH₂-phenyl. Ina class thereof R² is —H, —CH₃ or —CH₂-phenyl. phenyl.

In Embodiment 4 of this invention are compounds of Formula I, Embodiment2 or Embodiment 3, or any class thereof, or pharmaceutically acceptablesalts of the foregoing, wherein R⁵ and R⁶ are each independentlyselected from —C₁₋₄alkyl (for example both R⁵ and R⁶ are —CH₃), or R⁵and R⁶ are joined together with the carbon to which they are bothattached to form cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl. Ina class thereof, R⁵ and R⁶ are both —CH₃. In another class thereof, R⁵and R⁶ are joined together with the carbon to which they are bothattached to form cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl.

In Embodiment 5 of this invention are compounds of Formula I, FormulaIa, or any one of Embodiments 2, 3 or 4, or any class thereof, orpharmaceutically acceptable salts of the foregoing, wherein R³ is

(a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH,—CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂,

(b) —CH₂-phenyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl,

(c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl,

(d) phenyl or naphthyl, each unsubstituted or substituted with one tothree substituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl,

(e) —CH₂CH₂OCH₃, —CH₂CH₂CH₂OCH₃, —CH₂CH₂SCH₃, —CH₂CH₂CH₂SCH₃,—CH₂CH₂NHCH₃, —CH₂CH₂CH₂NHCH₃,

(f) pyridyl, unsubstituted or substituted with one to three substituentsindependently selected from fluoro chloro, bromo, —OR^(8a), —SH,—NR^(9a)R^(10a) or —C₁₋₃alkyl, or

(g) piperidinyl, pyrrolidinyl, tetrahydrofuranyl, or tetrahydropyranyl,each unsubstituted or substituted with one to three substituentsindependently selected from fluoro, chloro, bromo, —OR^(8a), —SH,—NR^(9a)R^(10a) or —C₁₋₃alkyl;

In a class of Embodiment 5, R³ is:

(a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH,—CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂,

(b) —CH₂-phenyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, or

(c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl.

In Embodiment 6 of this invention are compounds of Formula I, FormulaIa, or any one of Embodiments 2, 3, 4 or 5, or any class thereof, orpharmaceutically acceptable salts of the foregoing, wherein R⁴ is

(a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH,—CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂,

(b) —CH₂-phenyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl,

(c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl,

(d) phenyl or naphthyl, each unsubstituted or substituted with one tothree substituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl,

(e) —CH₂CH₂OCH₃, —CH₂CH₂CH₂OCH₃, —CH₂CH₂SCH₃, —CH₂CH₂CH₂SCH₃,—CH₂CH₂NHCH₃, —CH₂CH₂CH₂NHCH₃,

(f) pyridyl, unsubstituted or substituted with one to three substituentsindependently selected from fluoro chloro, bromo, —OR^(8b), —SH,—NR^(9b)R^(10b) or —C₁₋₃alkyl, or

(g) piperidinyl, pyrrolidinyl, tetrahydrofuranyl, or tetrahydropyranyl,each unsubstituted or substituted with one to three substituentsindependently selected from fluoro, chloro, bromo, —OR^(8b), —SH,—NR^(9b)R^(10b) or —C₁₋₃alkyl;

In a class of Embodiment 6, R⁴ is

(a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH,—CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂,

(b) —CH₂-phenyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, or

(c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to threesubstituents independently selected from fluoro, chloro, bromo,—OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl.

In Embodiment 7 of this invention are compounds of Formula Formula Ia,or any one of Embodiments 2, 3, 4, 5 or 6, or any class thereof, orpharmaceutically acceptable salts of the foregoing, wherein R³ and R⁴are each independently selected from —C₁₋₈alkyl,

—C₃₋₆cycloalkyl or —CH2-phenyl, and wherein each of R³ and R⁴ isunsubstituted or substituted as defined in Formula I. In class (A)thereof, R³ is selected from —C₁₋₈alkyl, —C₃₋₆cycloalkyl or —CH₂-phenyl.In class (B) thereof, R⁴ is selected from —C₁₋₈alkyl or —C₃₋₆cycloalkyl.In class (C) thereof, R³ is selected from —C₁₋₈alkyl, —C₃₋₆cycloalkyl or—CH₂-phenyl and R⁴ is selected from —C₁₋₈alkyl or —C₃₋₆cycloalkyl. Inclass (D) thereof, R³ and R⁴ are each unsubstituted. In class (E)thereof, R³ and R⁴ are each unsubstituted and independently selectedfrom —C₂₋₆alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or—CH₂-phenyl.

In Embodiment 8 are compounds of Formula I or a pharmaceuticallyacceptable salt thereof, wherein:

R¹ is —H or —CH₃;

R² is —H, —CH₃ or —CH₂-phenyl;

R³ is selected from —C₁₋₈alkyl, —C₃₋₆cycloalkyl or —CH₂-phenyl;

R⁴ is selected from —C₁₋₈alkyl or —C₃₋₆cycloalkyl; and

R⁵ and R⁶ are both —CH₃;

or R⁵ and R⁶ are joined together with the carbon to which they are bothattached to form —C₃₋₆cycloalkyl.

In a class of Embodiment 8, R³ is selected from —C₂₋₆alkyl,—C₃₋₆cycloalkyl or —CH2-phenyl, and R⁴ is selected from —C₂₋₆alkyl or—C₃₋₆cycloalkyl.

Reference to the compounds of Formula I herein encompasses the compoundsof Formula I and Ia, and all embodiments, classes and sub-classesthereof. The compounds of the invention encompass compounds of Formula Iand salts thereof when such salts are possible, includingpharmaceutically acceptable salts.

As used herein, “alkyl” refers to both branched- and straight-chainsaturated aliphatic hydrocarbon groups having the specified number ofcarbon atoms in a specified range. For example the term “C₁₋₁₀ alkyl”means linear or branched chain alkyl groups, including all possibleisomers, having 1, 2, 3, 4, 5, 7, 8, 9 or 10 carbon atoms, and includeseach of the decyl, nonyl, octyl, heptyl, hexyl and pentyl isomers aswell as n-, iso-, sec- and tent-butyl (butyl, s-butyl, i-butyl, t-butyl,Bu=butyl, collectively “—C₄alkyl”), n- and iso -propyl (propyl,i-propyl, Pr=propyl, collectively “—C₃alkyl”), ethyl (Et) and methyl(Me). “C₁₋₄alkyl” has 1, 2, 3 or 4 carbon atoms, and includes each ofn-, iso-, sec- and tert-butyl, n- and i-propyl, ethyl and methyl.

“Cycloalkyl” refers to a cyclized alkyl ring having the indicated numberof carbon atoms in a specified range. Thus, for example, “C₃₋₈cycloalkyl” encompasses each of cyclopropyl, cyclobutyl, cyclopentyl,cyclohexyl, cyloheptyl and cyclooctyl. “C₃₋₆cycloalkyl” encompasses eachof cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl. When cycloalkylis a substituent on an alkyl group in a compound of Formula I, thecycloalkyl substituent can be bonded to any available carbon in thealkyl group. The following are illustrations of —C₃₋₆cycloalkylsubstituents wherein the substituent is cyclopropyl in bold:

“Spiro-C₃₋₆cycloalkyl” refers to a cycloalkyl ring bonded to anon-terminal carbon atom wherein the non-terminal carbon atom is sharedwith the cycloalkyl group. Spiro-C₃₋₆cycloalkyl encompasses each ofspino-cyclopropyl, spiro-cyclobutyl, spiro -cyclopentyl andspiro-cyclohexyl. The following is an illustration of aSpiro-C₃₋₆cycloalkyl substituent wherein the substituent isspiro-cyclopropyl in bold:

Examples of —C₁₋₅alkyl—X—C₁₋₅alkyl groups include, but are not limitedto, —CH₂CH₂OCH₃, —CH₂CH₂CH₂OCH₃, —CH₂CH₂SCH₃, —CH₂CH₂CH₂SCH₃,—CH₂CH₂NHCH₃ or —CH₂CH₂CH₂NHCH₃.

“Aryl” refers to (i) phenyl, (ii) 9- or 10-membered bicyclic, fusedcarbocylic ring systems in which at least one ring is aromatic, and(iii) 11- to 14-membered tricyclic, fused carbocyclic ring systems inwhich at least one ring is aromatic. Suitable aryls include, forexample, substituted and unsubstituted phenyl and substituted andunsubstituted naphthyl. An aryl of particular interest is unsubstitutedor substituted phenyl.

“Heteroaryl” refers to (i) a 5- or 6-membered heteroaromatic ringcontaining from 1 to 4 heteroatoms independently selected from N, O andS, wherein each N is optionally in the form of an oxide, and (ii) a 9-or 10-membered bicyclic fused ring system, wherein the fused ring systemof (ii) contains from 1 to 6 heteroatoms independently selected from N,O and S, wherein each ring in the fused ring system contains zero, oneor more than one heteroatom, at least one ring is aromatic, each N isoptionally in the form of an oxide, and each S in a ring which is notaromatic is optionally S(O) or S(O)₂. Suitable 5- and 6-memberedheteroaromatic rings include, for example, pyridyl, 3-fluroropyridyl,4-fluoropyridyl, 3-methoxypyridyl, 4-methoxypyridyl, pyrrolyl,pyrazinyl, pyrimidinyl, pyridazinyl, triazinyl, thienyl, furanyl,imidazolyl, pyrazolyl, triazolyl (i.e., 1,2,3-triazolyl or1,2,4-triazolyl), tetrazolyl, oxazolyl, isooxazolyl, oxadiazolyl (i.e.,the 1,2,3-, 1,2,4-, 1,2,5- or 1,3,4-isomer), oxatriazotyl, thiazolyl,isothiazolyl, and thiadiazolyl. Suitable 9- and 10-memberedheterobicyclic, fused ring systems include, for example, benzofuranyl,indolyl, indazolyl, naphthyridinyl, isobenzofuranyl, benzopiperidinyl,benzisoxazolyl, benzoxazolyl, chromenyl, quinolinyl, isoquinolinyl,isoindolyl, benzopiperidinyl, benzofuranyl, imidazo[1,2-a]pyridinyl,benzotria.zolyl, indazolyl, indolinyl, and isoindolinyl. A class ofheteroaryls includes unsubstituted or substituted pyridyl or pyrimidyl,and particularly unsubstituted or substituted pyridyl.

The term “heterocyclic ring” refers to (i) a saturated 4- to 7-memberedcyclized ring and (ii) an unsaturated, non-aromatic 4 to 7-memberedcyclized ring comprised of carbon atoms and 1- 4 heteratomsindependently selected from O, N and S. Heterocyclic rings within thescope of this invention include, for example, azetidinyl, piperidinyl,morpholinyl, thiomorpholinyl, thiazolidinyl, isothiazolidinyl,oxazolidinyl, isoxazolidinyl, imidazolidinyl, piperazinyl,tetrahydrofuranyl, tetrahydrothienyl, pyrazolidinyl,hexahydropyrimidinyl, thiazinanyl, thiazepanyl, azepanyl, diazepanyl,tetrahydropyranyl, tetrahydrothiopyranyl, and dioxanyl. Examples of 4-to 7-membered, unsaturated, non -aromatic heterocyclic rings within thescope of this invention include mono-unsaturated heterocyclic ringscorresponding to the saturated heterocyclic rings listed in thepreceding sentence in which a single bond is replaced with a double bond(e.g., a carbon-carbon single bond is replaced with a carbon-carbondouble bond). A class of heterocyclic rings are 4 to 6-memberedsaturated nionocyclic rings comprised of carbon atoms and 1 or 2heteroatoms, wherein the heteroatoms are selected from N, O and S.Examples of 4 to 6 membered heterocyclic rings include but are notlimited to, azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl,morpholinyl, thiomorpholinyl, tetrahydrofuranyl, tetrahydropyranyl andtetrahydrothiopyranyl, and a sub-class thereof is piperidinyl,pyrrolidinyl, tetrahydrofuranyl or tetrahydropyranyl. The following isan illustration of R⁵ and R⁶ when they are joined together to form aheterocyclic ring:

With respect to substituents on a molecule, “geminally” “geminal” refersto two substituents, which may be the same or different, on one carbon.

It is understood that the specific rings and ring systems suitable foruse in the present invention are not limited to those listed in thepreceding paragraphs. These rings and ring systems are merelyrepresentative.

As would be recognized by one of ordinary skill in the art, certaincompounds of the present invention maybe able to exist as tautomers,tautomeric forms of these compounds, whether isolated individually or inmixtures, are within the scope of the present invention. For example, ininstances where an —OH substituent is permitted on a heteroaromatic ringand keto-enol tautomerism is possible, it is understood that thesubstituent might in fact be present, in whole or in part, in the oxo(═O) form.

A “stable” compound is a compound which can be prepared and isolated andwhose structure and properties remain or can be caused to remainessentially unchanged for a period of time sufficient to allow use ofthe compound for the purposes described herein (e.g., therapeutic orprophylactic administration to a subject). The compounds of the presentinvention are limited to stable compounds embraced by Formula I and itsembodiments. For example, certain moieties as defined in Formula I maybe unsubstituted or substituted, and the latter is intended to encompasssubstitution patterns (i.e., number and kind of substituents) that arechemically possible for the moiety and that result in a stable compound.

Each compound of Formula I is comprised of a phosphodiamide amino acidester having a defined (R) chiral center in the alkyl-ether linkinggroup connecting the nucleobase to the phosphorus as shown in Formula I,and may have one or more additional chiral centers depending onsubstituent selection. For example, each of Examples 1-52 herein alsohas an asymmetric phosphorus center. Accordingly, a compound of FormulaI may have multiple chiral centers (also referred to as asymmetric orstereogenic centers) This invention encompasses compounds having eitherthe (R) or (S) stereo-configuration at a phosphorus asymmetric centerand at any additional asymmetric centers that may be present in acompound of Formula I, as well as stereo-isomeric mixtures thereof.

This invention includes individual diastereomers, particularly epimers,i.e., compounds having the same chemical formula but which differ in thespatial arrangement around a single atom. This invention also includesmixtures of diastereomers, particularly mixtures of epimers, in allratios. Embodiments of this invention also include a mixture of epimersenriched with 51% or more of one of the epimers, including for example60% or more, 70% or more, 80% or more, or 90% or more of one epimer. Asingle epimer is preferred. An individual or single epimer refers to anepimer obtained by chiral synthesis and/or using generally knownseparation and purification techniques, and which may be 100% of oneepimer or may contain small amounts (e.g., 10% or less) of the oppositeepimer. Thus, individual diasteromers are a subject of the invention inpure form, both as levorotatory and as dextrorotatory antipodes, in theform of racemates and in the form of mixtures of the two diasteromers inall ratios. In the case of a cis/trans isomerism the invention includesboth the cis form and the trans form as well as mixtures of these formsin all ratios.

The preparation of individual stereoisomers can be carried out, ifdesired, by separation of a mixture by customary methods, for example bychromatography or crystallization, by the use of stereochemicallyuniform starting materials for the synthesis or by stereoselectivesynthesis. Optionally a derivatization can be carried out before aseparation of stereoisomers. The separation of a mixture ofstereoisomers can be carried out at an intermediate step during thesynthesis of a compound of Formula I or it can be done on a finalracemic product. Absolute stereochemistry may be determined by X-raycrystallography of crystalline products or crystalline intermediateswhich are derivatized, necessary, with a reagent containing astereogenic center of known configuration. Alternatively, absolutestereochemistry may be determined by Vibrational Circular Dichroism(VCD) spectroscopy analysis. The present invention includes all suchisomers, as well as salts, solvates (which includes hydrates) andsolvated salts of such racemates, enantiomers, diastereomers andtautomers and mixtures thereof.

The atoms in a compound of Fonnula I. may exhibit their natural isotopicabundances, or one or more of the atoms may be artificially enriched ina particular isotope having the same atomic number, but an atomic massor mass number different from the atomic mass or mass numberpredominantly found in nature. The present invention is meant to includeall suitable isotopic variations of the compounds of Formula I; forexample, different isotopic forms of hydrogen (H) include protium (¹H)and deuterium (²H). Protium is the predominant hydrogen isotope found innature. Enriching for deuterium may afford certain therapeuticadvantages, such as increasing in vivo half-life or reducing dosagerequirements, or may provide a compound useful as a standard forcharacterization of biological samples. Isotopically-enriched compoundsof Formula I can be prepared without undue experimentation byconventional techniques well known to those skilled in the art or byprocesses analogous to those described in the Schemes and. Examplesherein using appropriate isotopically-enriched reagents and/orintermediates.

The compounds can be administered in the form of pharmaceuticallyacceptable salts. The term “pharmaceutically acceptable salt” refers toa salt which is not biologically or otherwise undesirable (e.g., isneither toxic nor otherwise deleterious to the recipient thereof), Sincethe compounds of Formula I contain by definition at least one basicgroup, the invention includes the corresponding pharmaceuticallyacceptable salts. When the compounds of Formula I contain one or moreacidic groups, the invention also includes the correspondingpharmaceutically acceptable salts. Thus, the compounds of Formula I thatcontain acidic groups (e.g., —COOH) can be used according to theinvention as, for example but not limited to, alkali metal salts,alkaline earth metal salts or as ammonium salts. Examples of such saltsinclude but are not limited to sodium salts, potassium salts, calciumsalts, magnesium salts or salts with ammonia or organic amines such as,for example, ethylamine, ethanolamine, triethanolamine or amino acids,Compounds of Formula I, which contain one or more basic groups, i.e.groups which can be protonated, can be used according to the inventionin the form of their acid addition salts with inorganic or organic acidsas, for example but not limited to, salts with hydrogen chloride,hydrogen bromide, phosphoric acid, sulfuric acid, nitric acid,benzenesulfonic acid, methanesulfonic acid, p -toluenesulfonic acid,naphthalenedisulfonic acids, oxalic acid, acetic acid, trifluoroaceticacid, tartaric acid, lactic acid, salicylic acid, benzoic acid, formicacid, propionic acid, pivalic acid, diethylacetic acid, malonic acid,succinic acid, pimelic acid, fumaric acid, maleic acid, malic acid,sulfatninic acid, phenylpropionic acid, gluconic acid, ascorbic acid,isonicotinic acid, citric acid, adipic acid, etc. If the compounds ofFormula I simultaneously contain acidic and basic groups in the moleculethe invention also includes, in addition to the salt forms mentioned,inner salts or betaines (zwitterions). Salts can be obtained from thecompounds of Formula I by customary methods which are known to theperson skilled in the art, for example by combination with an organic orinorganic acid or base in a solvent or dispersant, or by anion exchangeor cation exchange from other salts. The present invention also includesall salts of the compounds of Formula I which, owing to lowphysiological compatibility, are not directly suitable for use inpharmaceuticals but which can be used, for example, as intermediates forchemical reactions or for the preparation of pharmaceutically acceptablesalts.

The instant invention encompasses any composition comprised of acompound of Formula I or a compound that is a salt thereof, includingfor example but not limited to, a composition comprised of said compoundassociated together with one or more additional molecular and/or ioniccomponent(s) which may be referred to as a “co-crystal.” The term“co-crystal” as used herein refers to a solid phase (which may or maynot be crystalline) wherein two or more different molecular and/or ioniccomponents (generally in a stoichiometric ratio) are held together bynon-ionic interactions including but not limited to hydrogen-bonding,dipole-dipole interactions, dipole-quadrupole interactions or dispersionforces (van der Waals). There is no proton transfer between thedissimilar components and the solid phase is neither a simple salt nor asolvate. A discussion of co-crystals can be found, e.g., in S.Aitipamula et al., Crystal Growth and Design, 2012, 12 (5), pp.2147-2152.

More specifically with reference to this invention, a co-crystal iscomprised of a compound of Formula I or a pharmaceutically acceptablesalt thereof, and one or more non-pharmaceutically active component(s)which is not biologically or otherwise undesirable (e.g., is neithertoxic nor otherwise deleterious to the recipient thereof). Co -crystalscan be obtained from a compound of Formula I, or a pharmaceuticallyacceptable salt thereof, by customary methods known in the chemicalarts. For example, co-crystals comprised of a compound of this inventioncould be prepared by adding an acid or a neutral molecule at the desiredstoichiometry to the compound, adding an appropriate solvent to achievedissolution and, for example, precipitating, lyophilizing orconcentrating the solution to obtain the solid composition. Theco-crystal can be, but is not limited to, an embodiment wherein thecomposition is comprised of a neutral compound (i.e. not a salt form) ofFormula I and one or more non-pharmaceutically active component(s); andin a further embodiment, the co-crystal composition is crystalline.Crystalline compositions may be prepared, for example, by adding an acidor a neutral molecule at the desired stoichiometry to the compound ofFormula I, adding an appropriate solvent and heating to achieve completedissolution, and then allowing the solution to cool and the crystals togrow. The present invention also includes all co-crystals of thecompounds of this invention which, owing to low physiologicalcompatibility, are not directly suitable for use in pharmaceuticals butwhich can be used, for example, as intermediates for chemical reactionsor for the preparation of pharmaceutically acceptable co-crystals orsalts.

Compounds of the present invention may exist in amorphous form and/orone or more crystalline forms, and as such all amorphous and crystallineforms and mixtures thereof of the compounds of Formula I and saltsthereof are intended to be included within the scope of the presentinvention. In addition, some of the compounds of the instant inventionmay form solvates with water (i.e., a hydrate) or common organicsolvents. Such solvates and hydrates, particularly the pharmaceuticallyacceptable solvates and hydrates, of the compounds of this invention arelikewise encompassed within the scope of the compounds defined byFormula I the pharmaceutically acceptable salts thereof, along withun-solvated and anhydrous forms of such compounds.

Accordingly, the compounds of Formula I, embodiments thereof andspecific compounds described and claimed herein encompass all possiblepharmaceutically acceptable salts, stereoisomers, tautomers, physicalforms (e.g., amorphous and crystalline forms), co-crystal forms, solvateand hydrate forms and any combination of the foregoing forms where suchforms are possible.

The compounds of Formula I described herein are prodrugs. A discussionof prodrugs is provided in (a) Stella, V. J.; Borchardt, R. T.; Hageman,M. J.; Oliyai, R.; Maag, H. et al. Prodrugs: Challenges and Rewards Part1 and Part 2 ; Springer, p. 726: New York, N.Y., USA, 2007, (b) Rautio,J.; Kumpulainen, H.; Heimbach, T.; Oliyai, R.; Oh, D. et al. Prodrugs:design and clinical applications. Nat. Rev. Drug Discov. 2008, 7, 255,(c) T. Higuchi and V. Stella, Pro-drugs as Novel Delivery Systems (1987)14 of the A.C.S. Symposium Series, and in (d) Bioreversible Carriers inDrug Design, (1987) Edward B. Roche, ed., American PharmaceuticalAssociation and Pergamon Press. More specifically, compounds of FormulaI (and pharmaceutically acceptable salts thereof) are prodrugmodifications of tenofovir, which is a mono-phosphonate. The compoundsof Formula I may be converted intracellularly (in vivo or in vitro) tothe corresponding monophosphate or diphosphate of tenofovir. Theconversion may occur by one or more mechanisms, e.g., anenzyme-catalyzed chemical reaction, a metabolic chemical reaction,and/or a spontaneous chemical reaction (e.g., solvolysis), such as, forexample, through hydrolysis in blood. While not wishing to be bound byany particular theory, tenofovir diphosphate is generally understood tobe responsible for inhibiting the HIV RT enzyme and for the resultingantiviral activity after administration of the compound of Formula I toa subject.

Another embodiment of the present invention is a compound of Formulawherein the compound or its salt is in a substantially pure form. Asused herein “substantially pure” means suitably at least about 60 wt. %,typically at least about 70 wt. %, preferably at least about 80 wt. %,more preferably at least about 90 wt. % (e.g., from about 90 wt. % toabout 99 wt. %), even more preferably at least about 95 wt. % (e.g.,from about 95 wt. % to about 99 wt. %, or from about 98 wt. % to 100 wt.%), and most preferably at least about 99 wt. % (e.g., 100 wt. %) of aproduct containing a compound of Formula I or its salt (e.g., theproduct isolated from a reaction mixture affording the compound or salt)consists of the compound or salt. The level of purity of the compoundsand salts can be determined using a standard method of analysis such as,high performance liquid chromatography, and/or mass spectrometry or NMRtechniques. If more than one method of analysis is employed and themethods provide experimentally significant differences in the level ofsS purity determined, then the method providing the highest purity levelgoverns. A compound or salt of 100% purity is one which is free ofdetectable impurities as determined by a standard method of analysis.With respect to a compound of Formula I which has one or more asymmetriccenters and can occur as mixtures of stereoisomers, a substantially purecompound can be either a substantially pure mixture of the stereoisomersor a substantially pure individual stereoisomer.

The compounds of Formula I herein, and pharmaceutically acceptable saltsthereof, are useful for HIV reverse transcriptase inhibition and forinhibiting HIV replication in vitro and in vivo, More particularly, thecompounds of Formula I for inhibiting the polymerase function of HIV-1reverse transcriptase. The testing of compounds of the Examples of thisinvention in the Viking assay set forth in Example 53 below, illustratethe ability of compounds of the invention to inhibit the RNA-dependentDNA polymerase activity of HIV-1 reverse transcriptase. The compounds ofFormula I may also be useful agents against HIV-2. The compounds of ofExamples 1-52 of the present invention may also exhibit activity againstdrug resistant forms of HIV (e.g., NNRTI-associated mutant strains K103Nand/or Y181C; NRTI-associated mutant strains M184V and M184I mutants.

This invention also encompasses methods for the treatment or prophylaxisof infection by HIV, for the inhibition of HIV reverse transcriptase,for the treatment, prophylaxis, or delay in the onset of AIDS in asubject in need thereof, which comprise administering to the subject aneffective amount of a compound of the invention or a pharmaceuticallyacceptable salt thereof.

The invention further encompasses methods for the treatment orprophylaxis of infection by HIV, for the inhibition of HIV reversetranscriptase, for the treatment, prophylaxis, or delay in the onset ofAIDS in a subject in need thereof, which comprise administering to thesubject an effective amount of a compound of the invention or apharmaceutically acceptable salt thereof in combination with aneffective amount of one or more additional anti-HIV agents selected fromthe group consisting of HIV antiviral agents, immunomodulators, andanti-infective agents. Within this embodiment, the anti-HIV agent is anantiviral selected from the group consisting of HIV protease inhibitors,HIV reverse transcriptase inhibitors, HIV integrase inhibitors, HIVfusion inhibitors, HIV entry inhibitors, and HIV maturation inhibitors

The invention encompasses a pharmaceutical composition comprising aneffective amount of a compound of the invention, or a pharmaceuticallyacceptable salt thereof, and a pharmaceutically acceptable carrier. Theinvention also encompasses a pharmaceutical composition comprising aneffective amount of a compound of the invention, or a pharmaceuticallyacceptable salt thereof, and a pharmaceutically acceptable carrierfurther comprising an effective amount of one or more additionalanti-HIV agents selected from the group consisting of HIV antiviralagents, immunomodulators, and anti-infective agents. Within thisembodiment, the anti-HIV agent is an antiviral selected from the groupconsisting of HIV protease inhibitors, HIV reverse transcriptaseinhibitors, HIV integrase inhibitors, HIV fusion inhibitors, HIV entryinhibitors, and HIV maturation inhibitors.

The compounds of this invention could also be useful for inhibition ofHBV reverse transcriptase. Accordingly, this invention also encompassesmethods for the treatment of chronic hepatitis B which compriseadministering to the subject an effective amount of a compound of theinvention or a pharmaceutically acceptable salt thereof.

The invention also encompasses a compound of the invention, or apharmaceutically acceptable salt thereof, for use in the preparation ofa medicament for the treatment or prophylaxis of infection by HIV, forthe inhibition of HIV reverse transcriptase, or for the treatment,prophylaxis, or delay in the onset of AIDS in a subject in need thereof.

Other embodiments of the present invention include the following:

(a) A pharmaceutical composition comprising an effective amount of acompound of Formula I or a pharmaceutically acceptable salt thereof, anda pharmaceutically acceptable carrier.

(b) A pharmaceutical composition which comprises the product prepared bycombining (e.g., mixing) an effective amount of a compound of Formula Ior a pharmaceutically acceptable salt thereof, and a pharmaceuticallyacceptable carrier.

(c) The pharmaceutical composition of (a) or (b), further comprising aneffective amount of one or more an anti-HIV agents selected from thegroup consisting of HIV antiviral agents, immunomodulators, andanti-infective agents.

(d) The pharmaceutical composition of (c), wherein the anti-HIV agent isselected from one or more of an antiviral selected from the groupconsisting of HIV protease inhibitors, nucleoside HIV reversetranscriptase inhibitors, non-nucleoside HIV reverse transcriptaseinhibitors, HIV integrase inhibitors, HIV fusion inhibitors, HIV entryinhibitors and HIV maturation inhibitors.

(e) A combination which is (i) a compound of Formula I or apharmaceutically acceptable salt thereof and (ii) an anti-HIV agentselected from the group consisting of HIV antiviral agents,immunomodulators, and anti-infective agents; wherein the compound andthe anti-HIV agent are each employed in an amount that renders thecombination effective for inhibition of HIV reverse transcriptase, fortreatment or prophylaxis of infection by HIV, or for treatment,prophylaxis of, or delay in the onset or progression of AIDS.

The combination of (e), wherein the anti-HIV agent is an antiviralselected from the group consisting of HIV protease inhibitors,nucleoside HIV reverse transcriptase inhibitors, non-nucleoside HIVreverse transcriptase inhibitors, HIV integrase inhibitors, HIV fusioninhibitors, HIV entry inhibitors and HIV maturation inhibitors.

(g) A method for the inhibition of HIV reverse transcriptase in asubject in need thereof which comprises administering to the subject aneffective amount of a compound of Formula I or pharmaceuticallyacceptable salt thereof.

(h) A method for the prophylaxis or treatment of infection by HIV (e.g.,HIV-1) in a subject in need thereof which comprises administering to thesubject an effective amount of a compound of Formula I orpharmaceutically acceptable salt thereof.

(i) The method of (h), wherein the compound of Formula I or apharmaceutically acceptable salt thereof is administered in combinationwith an effective amount of at least one other HIV antiviral selectedfrom the group consisting of HIV protease inhibitors, HIV integraseinhibitors, non-nucleoside HIV reverse transcriptase inhibitors,nucleoside HIV reverse transcriptase inhibitors, HIV fusion inhibitors,HIV entry inhibitors and HIV maturation inhibitors.

(j) A method for the prophylaxis, treatment or delay in the onset orprogression of AIDS in a subject in need thereof which comprisesadministering to the subject an effective amount of a compound ofFormula I or pharmaceutically acceptable salt thereof

(k) The method of (j), wherein the compound is administered incombination with an effective amount of at least one other HIV antiviralselected from the group consisting of HIV protease inhibitors, HIVintegrase inhibitors, non-nucleoside HIV reverse transcriptaseinhibitors, nucleoside HIV reverse transcriptase inhibitors, HIV fusioninhibitors, HIV entry inhibitors and HIV maturation inhibitors.

(l) A method for the inhibition of HIV reverse transcriptase in asubject in need thereof which comprises administering to the subject thepharmaceutical composition of (a), (b), (c) or (d) or the combination of(e) or (f).

(m) A method for the prophylaxis or treatment of infection by HIV (e.g.,HIV-1) in a subject in need thereof, which comprises administering tothe subject the pharmaceutical composition of (a), (b), (c) or (d) orthe combination of (e) or (f).

(n) A method for the prophylaxis, treatment, or delay in the onset orprogression of AIDS in a subject in need thereof which comprisesadministering to the subject the pharmaceutical composition of (a), (b),(c) or (d) or the combination of (e) or (f).

The present invention also includes a compound of Formula I orpharmaceutically acceptable salt thereof (i) for use in, (ii) for use asa medicament for, or (iii) for use in the preparation of a medicamentfor: (a) therapy (e.g., of the human body), (b) medicine, (c) inhibitionof HIV reverse transcriptase, (d) treatment or prophylaxis of infectionby HIV, or (e) treatment, prophylaxis of, or delay in the onset orprogression of AIDS. In these uses, the compounds of the presentinvention can optionally be employed in combination with one or moreanti-HIV agents selected from HIV antiviral agents, anti -infectiveagents, and immunomodulators.

Additional embodiments of the invention include the pharmaceuticalcompositions, combinations and methods set forth in (a)-(n) above andthe uses (i)(a)-(e) through (iii)(a)-(e) set forth in the precedingparagraph, wherein the compound of the present invention employedtherein is a compound of one of the embodiments, aspects, classes,sub-classes, or features described above. In all of these embodimentsetc., the compound may optionally be used in the form of apharmaceutically acceptable salt.

Additional embodiments of the present invention include each of thecompounds, pharmaceutical compositions, combinations, methods and usesset forth in the preceding paragraphs, wherein the compound or its saltemployed therein is substantially pure. With respect to a pharmaceuticalcomposition comprising a compound of Formula I or its salt and apharmaceutically acceptable carrier and optionally one or moreexcipients, it is understood that the term “substantially pure” is inreference to a compound of Formula I or its salt per se.

Still additional embodiments of the present invention include thepharmaceutical compositions, combinations and methods set forth in(a)-(n) above and the uses (i)(a)-(e) through (iii)(a)-(e) set forthabove, wherein the HIV of interest is HIV-1. Thus, for example, in thepharmaceutical composition (d), the compound of Formula I is employed inan amount effective against HIV-1 and the anti-HIV agent is an HIV-1antiviral selected from the group consisting of HIV-1 proteaseinhibitors, HIV-1 reverse transcriptase inhibitors, HIV-1 integraseinhibitors, HIV-1 fusion inhibitors, HIV-1 entry inhibitors and HIV-1maturation inhibitors. The compounds of Formula I may also be usefulagents against HIV-2.

The term “administration” and variants thereof (e.g., “administering” acompound) in reference to a compound of Fonnula I means providing thecompound to the individual in need of treatment or prophylaxis andincludes both self-administration and administration to the patient byanother person. When a compound is provided in combination with one ormore other active agents (e.g., antiviral agents useful for treating orprophylaxis of HIV infection or AIDS), “administration” and its variantsare each understood to include provision of the compound and otheragents at the same time or at different times. When the agents of acombination are administered at the same time, they can be administeredtogether in a single composition or they can be administered separately.

As used herein, the term “composition” is intended to encompass aproduct comprising the specified ingredients, as well as any productwhich results from combining the specified ingredients. Ingredientssuitable for inclusion in a pharmaceutical composition arepharmaceutically acceptable ingredients, which means the ingredientsmust be compatible with each other and not deleterious to the recipientthereof.

The term “subject” or “patient” as used herein refers to an animal,preferably a mammal, most preferably a human, who has been the object oftreatment, observation or experiment.

The term “effective amount” as used herein means an amount of a compoundsufficient to inhibit HIV reverse transcriptase, inhibit HIVreplication, exert a prophylactic effect, and/or a exert a therapeuticeffect after administration. One embodiment of “effective amount” is a“therapeutically effective amount” which is an amount of a compound thatis effective for inhibiting HIV reverse transcriptase, inhibiting HIVreplication (either of the foregoing which may also be referred toherein as an “inhibition effective amount”), treating HIV infection,treating AIDS, delaying the onset of AIDS, and/or slowing progression ofARC or AIDS in a patient infected with HIV. Another embodiment of“effective amount” is a “prophylactically effective amount” which is anamount of the compound that is effective for prophylaxis of I-HVinfection in a subject not infected with HIV, or prophylaxis of ARC orAIDS in an HIV-infected patient. It is understood that an effectiveamount can simultaneously be both a therapeutically effective amount,e.g., for treatment of HIV infection, and a prophylactically effectiveamount, e.g., for prevention or reduction of risk for developing AIDS ina subject infected with HIV. The term “preventing,” as used herein withrespect to an HIV viral infection or AIDS, refers to reducing thelikelihood or severity of HIV infection or AIDS. When the compound ofFormula I is administered as a salt, reference to an amount of thecompound in milligrams or grams is based on the free form (i.e., thenon-salt form) of the compound. In the combination therapies of thepresent invention, an effective amount can refer to each individualagent or to the combination as a whole, wherein the amounts of allagents administered in the combination are together effective, butwherein a component agent of the combination may or may not be presentindividually in an effective amount with reference to what is consideredeffective for that component agent if it were administered alone.

In the method of the present invention (i.e., inhibiting HIV reversetranscriptase, treating or prophylaxis of MV infection, inhibiting HIVreplication, treating or prophylaxis of AIDS, delaying the onset ofAIDS, or delaying or slowing progression of AIDS), the compounds of thisinvention, optionally in the form of a salt, can be administered bymeans that produce contact of the active agent with the agent's site ofaction. They can be administered by conventional means available for usein conjunction with pharmaceuticals, either as individual therapeuticagents or in a combination of therapeutic agents. They can beadministered alone, but typically are administered with a pharmaceuticalcarrier selected on the basis of the chosen route of administration andstandard pharmaceutical practice. The compounds of the invention can,for example, be administered orally (e.g., via tablet or capsule),parenterally (including subcutaneous injections, intravenous,intramuscular or intrasternal injection, or infusion techniques), byinhalation spray, or rectally, in the form of a unit dosage of apharmaceutical composition containing an effective amount of thecompound and conventional non-toxic pharmaceutically acceptablecarriers, adjuvants and. vehicles. The compound could also beadministered via an implantable drug delivery device adapted to providean effective amount of the compound or a pharmaceutical composition ofthe compound over an extended period of time for example, but notlimited to, over the course of a month, 3months, 6 months or a year.

Solid preparations suitable for oral administration (e.g., powders,pills, capsules and tablets) can be prepared according to techniquesknown in the art and can employ such solid excipients as starches,sugars, kaolin, lubricants, binders, disintegrating agents and the like.Liquid preparations suitable for oral administration (e.g., suspensions,syrups, elixirs and the like) can be prepared according to techniquesknown in the art and can employ any of the usual media such as water,glycols, oils, alcohols and the like.

Parenteral compositions can be prepared according to techniques known inthe art and typically employ sterile water as a carrier and optionallyother ingredients, such as a solubility aid, Injectable solutions can beprepared according to methods known in the art wherein the carriercomprises a saline solution, a glucose solution or a solution containinga mixture of saline and glucose. Implantable compositions can beprepared according to methods known in the art wherein the carriercomprises the active chemical ingredient with polymers as suitableexcipients, or utilizing an implantable device for drug delivery.Further description of methods suitable for use in preparingpharmaceutical compositions for use in the present invention and ofingredients suitable for use in said compositions is provided inRemington's Pharmaceutical Sciences, 18th edition, edited by A. R.Gennaro, Mack Publishing Co., 1990 and in Remington—The Science andPractice of Pharmacy, 22nd Edition, published by Pharmaceutical Pressand Philadelphia College of Pharmacy at University of the Sciences,2012, ISBN 978 0 85711-062-6 and prior editions.

Formulations of compounds described by Formula I that result in drugsupersaturation and/or rapid dissolution may be utilized to facilitateoral drug absorption. Formulation approaches to cause drugsupersaturation and/or rapid dissolution include, but are not limitedto, nanoparticulate systems, amorphous systems, solid solutions, soliddispersions, and lipid systems. Such formulation approaches andtechniques for preparing them are known in the art. For example, soliddispersions can be prepared using excipients and processes as describedin reviews (e.g., A. T. M. Serajuddin, J Pharm Sci, 88:10, pp. 1058-1066(1999)). Nanoparticulate systems based on both attrition and directsynthesis have also been described in reviews such as Wu et al (F.Kesisoglou, S. Panmai, Y. Wu, Advanced Drug Delivery Reviews, 59:7 pp.631-644 (2007)).

The compounds of Formula I can be administered in a dosage range of0.001 to 1000 mg/kg of mammal (e.g., human) body weight per day, or atlonger time intervals on non-consecutive days as appropriate, in asingle dose or in divided doses. One example of a dosage range is 0,01to 500 mg/kg body weight per day, or at other time intervals asappropriate, administered orally or via other routes of administrationin a single dose or in divided doses. Another example of a dosage rangeis 0.1 to 100 mg/kg body weight per day, or at other time intervals asappropriate, administered orally or via other routes of administrationin single or divided doses. Another example of a dosage range is 50 mgto 1 gram per day, in a single dose or divided doses.

Daily or weekly administration or less frequent dosing regimens withlonger time intervals on non-consecutive days (as discussed below), canbe via any suitable route of administration, e.g. but not limited to,oral or parenteral. Daily or weekly administration is preferably viaoral administration. For either a daily or weekly dosing regimen, oneach day (calendar day or about a 24 hour period of time) of drugadministration (the “administration day”), the desired dosage amount maybe administered once per administration day or in divided dosage amountsadministered at two or more staggered times during the administrationday, e.g., a first administration followed about 12 hours later with asecond administration during the course of an administration day (the“dosage time(s)”). The desired dosage amount at each of the one or moredosage times on an administration day can be administered via one oraldosage unit such as a tablet, or more than one oral dosage unit asappropriate. Preferably the administration is via a single oral dosageunit, e.g. a tablet, once per administration day.

For weekly or less frequent dosing regimens with longer time intervalson non-consecutive days, a parenteral route of administration may beemployed. Examples of dosing regimens with longer time intervals onnon-consecutive days include but are not limited to administrationweekly (every seventh day with leeway as to exact date of dosing),bi-weekly (every two weeks with leeway as to exact date of dosing),monthly (e.g., every 30 days, or the same calendar day each month withleeway as to exact date of dosing), bimonthly (e.g., every 60 days, orthe same calendar day every two months with leeway as to exact date ofdosing), every 3 months (e.g., every 90 days, or the same calendar dayevery three months with leeway as to exact date of dosing), every sixmonths (e.g., every 180 days, or the same calendar day every six monthswith leeway as to exact date of dosing), or yearly (e.g., every 12months with leeway as to exact date of the annual dosing). “Leeway” isintended to mean that the dosing regimens described herein alsoencompasses those wherein the patient generally follows the timeintervals between administration days including when the interval is notalways strictly followed by the patient, e.g., a weekly dosing regimenwhere the patient may take the drug product the day before or the dayafter the seventh day following the prior administration day for one ormore weeks. The leeway time may increase as the dosing regimen intervalincreases. For oral (e.g., tablets or capsules) or other routes ofadministration, the dosage units may contain 1.0 mg to 1000 mg of theactive ingredient, for example but not limited to, 1., 5, 10, 15, 20,25, 50, 75, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800,900 or 1000 milligrams of the active ingredient for the symptomaticadjustment of the dosage to the patient to be treated. Furthermore, thecompound may be formulated in oral formulations for immediate ormodified release such as extended or controlled release.

The favorable pharmacokinetic profile of tested compounds of thisinvention may also render the the compounds suitable for less frequentdosing. Thus, the compounds of the invention could be administeredorally, weekly or parenterally at longer time intervals as describedabove. For parenteral administration, the compositions can beadministered, e.g., intravenously (IV) or intramuscularly (IM) viainjection, or using other infusion techniques. One or more of suchinjections or infusions may be administered at each dosing time intervalas needed to deliver the appropriate amount of active agent. Thecompound. could also be administered parenterally using an implantabledevice. For parenteral administration including implantable devicesemploying longer duration dosing intervals such as once monthly, onceevery 3 months, once every 6 months, once yearly or longer intervals,the dosage amount would be adjusted upward as needed to provideeffective treatment during the time intervals between administration ofeach dose.

The specific dose level and frequency of dosage for any particularpatient may be varied and will depend upon a variety of factorsincluding the activity of the specific compound employed, the metabolicstability and length of action of that compound, the age, body weight,general health, sex, diet, mode and time of administration, rate ofexcretion, drug combination, the severity of the particular condition,and the host undergoing therapy. In some cases, depending on the potencyof the compound or the individual response, it may be necessary todeviate upwards or downwards from the given dose. The amount andfrequency of administration will be regulated according to the judgmentof the attending clinician considering such factors.

As noted above, the present invention is also directed to use of acompound of Formula I with one or more anti-HIV agents. An “anti-HIVagent” is any agent which is directly or indirectly effective in theinhibition of HIV, the treatment or prophylaxis of HIV infection, and/orthe treatment, prophylaxis or dday in the onset or progression of AIDS.It is understood that an anti-HIV agent is effective in treating,preventing, or delaying the onset or progression of HIV infection orAIDS and/or diseases or conditions arising therefrom or associatedtherewith. For example, the compounds of this invention may beeffectively administered, whether at periods of pre-exposure and/orpost-exposure, in combination with effective amounts of one or moreanti-HIV agents selected from HIV antiviral agents, imunomodulators,antiinfectives, or vaccines useful for treating HIV infection or AIDS.Suitable HIV antivirals for use in combination with the compounds of thepresent invention include, for example, those listed in Table A asfollows:

TABLE A Antiviral Agents for Treating HIV infection or AIDS Name Typeabacavir, ABC, Ziagen ® nRTI abacavir + lamivudine, Epzicom ® nRTIabacavir + lamivudine + zidovudine, Trizivir ® nRTI amprenavir,Agenerase ® PI atazanavir, Reyataz ® PI AZT, zidovudine, azidothymidine,Retrovir ® nRTI capravirine nnRTI darunavir, Prezista ® PI ddC,zalcitabine, dideoxycytidine, Hivid ® nRTI ddI, didanosine,dideoxyinosine, Videx ® nRTI ddI (enteric coated), Videx EC ® nRTIdelavirdine, DLV, Rescriptor ® nnRTI dolutegravir, Tivicay ® InIdoravirine, MK-1439 nnRTI efavirenz, EFV, Sustiva ®, Stocrin ® nnRTIefavirenz + emtricitabine + tenofovir DF, Atripla ® nnRTI + nRTI EFdA(4′-ethynyl-2-fluoro-2′-deoxyadenosine) nRTI Elvitegravir InIemtricitabine, FTC, Emtriva ® nRTI emtricitabine + tenofovir DF,Truvada ® nRTI emvirine, Coactinon ® nnRTI enfuvirtide, Fuzeon ® FIenteric coated didanosine, Videx EC ® nRTI etravirine, TMC-125 nnRTIfosamprenavir calcium, Lexiva ® PI indinavir, Crixivan ® PI lamivudine,3TC, Epivir ® nRTI lamivudine + zidovudine, Combivir ® nRTI lopinavir PIlopinavir + ritonavir, Kaletra ® PI maraviroc, Selzentry ® EInelfinavir, Viracept ® PI nevirapine, NVP, Viramune ® nnRTI PPL-100(also known as PL-462) (Ambrilia) PI raltegravir, MK-0518, Isentress ™InI Rilpivirine nnRTI ritonavir, Norvir ® PI saquinavir, Invirase ®,Fortovase ® PI stavudine, d4T, didehydrodeoxythymidine, Zerit ® nRTItipranavir, Aptivus ® PI vicriviroc EI EI = entry inhibitor; FI = fusioninhibitor; InI = integrase inhibitor; PI = protease inhibitor; nRTI =nucleoside reverse transcriptase inhibitor; nnRTI = non-nucleosidereverse transcriptase inhibitor. Some of the drugs listed in the tableare used in a salt form; e.g., abacavir sulfate, delavirdine mesylate,indinavir sulfate, atazanavir sulfate, nelfinavir mesylate, saquinavirmesylate.

It is understood that the scope of combinations of the compounds of thisinvention with anti-I-IIV agents is not limited to the HIV antiviralslisted in Table A, but includes in principle any combination with anypharmaceutical composition useful for the treatment or prophylaxis ofAIDS. The HIV antiviral agents and other agents will typically beemployed in these combinations in their conventional dosage ranges andregimens as reported in the art, including, for example, the dosagesdescribed in the Physicians' Desk Reference, Thomson PDR, Thomson PDR,57th edition (2003), the 58th edition (2004), or the 59th edition (2005)and the current Physicians' Desk Reference (68th ed.). (2014), Montvale,N.J.: PDR Network. The dosage ranges for a compound of the invention inthese combinations can be the same as those set forth above.

The compounds of this invention are also useful in the preparation andexecution of screening assays for antiviral compounds. For example, thecompounds of this invention are useful for isolating enzyme mutants,which are excellent screening tools for more powerful antiviralcompounds. Furthermore, the compounds of this invention are useful inestablishing or determining the binding site of other antivirals to HIVreverse transcriptase, e.g., by competitive inhibition.

The following non-exclusive list of solvents, reagents, protectinggroups, moieties, and other designations may be referred to by theirabbreviations:

Ac Acetyl aq Aqueous AUC Area under the curve Bu Butyl Bz Benzoyl DBU1,8-diazabicyclo-[5.4.0]undec-7-ene DCM dichloromethane DIEA or Hünig'sbase N,N-diisopropylethylamine DMF dimethylformamide DMSO dimethylsulfoxide Et Ethyl EtOH Ethanol EtOAc ethyl acetate g Grams h Hour HIVhuman immunodeficiency virus HPBCD hydroxypropyl β-cyclodextrin HPLChigh-performance liquid chromatography Hz hertz IPA isopropanol IVintravenous iPr isopropyl L liter LC liquid chromatography LC/MS liquidchromatography mass spectrometry Me methyl MeOH methanol mg milligramsMHz megahertz min minute μL microliters mL milliliters mmol millimolesMS mass spectrometry NHS normal human serum NMR nuclear magneticresonance spectroscopy PBMC peripheral blood mononuclear cell PEPetroleum ether Ph phenyl P.O. oral Pr propyl RT or rt room temperature(ambient, about 25° C.) sat or sat'd saturated SFC supercritical fluidchromatography tBu tert-butyl TEA triethylamine (Et₃N) TFAtrifluoroacetic acid TFV Tenofovir TFV-MP Tenofovir monophosphoateTFV-DP Tenofovir diphosphate THF tetrahydrofuran TMS tetramethylsilaneUPLC ultra high pressure liquid chromatography UV ultraviolet UV/VISultraviolet/visible

Several methods for preparing the compounds of this invention aredescribed in the following Schemes and Examples. Starting materials andintermediates are purchased or are made using known procedures, or asotherwise illustrated. Some frequently applied routes to the compoundsof Formula I are described in the Schemes that follow. In some cases theorder of carrying out the reaction steps in the schemes may be varied tofacilitate the reaction or to avoid unwanted reaction products.

Several methods for preparing the compounds of this invention are alsodescribed in the Examples. Starting materials and intermediates werepurchased commercially from common catalog sources or were made usingknown procedures, or as otherwise illustrated.

Intermediates compounds of Formula S-1 are prepared from(R)-(((1-(6-amino-9H -purin-9-yl)propan-2-yl)oxy)methyl)phosphonic acid,referred to herein as TFV, with variably-substituted phenols in aone-step one-pot condensation reaction with 2,2′-dipyridyidisulfide(aldrithiol), triphenylphosphine, and base, with p-chlorophenol and m-cyanophenol being preferred. Amino esters that are not commerciallyavailable can be readily prepared by condensation between thecorresponding amino acid and alcohols with thionyl chloride. Thesubsequent reaction of S-1 with a corresponding glycolic ester or lacticester in the presence of DBU base yields the products of Formula S-2 ofthe present invention.

Reactions sensitive to moisture or air were performed under nitrogen orargon using anhydrous solvents and reagents. The progress of reactionswas determined by either analytical thin layer chromatography (TLC)usually performed with E. Merck pre-coated TLC plates, silica gel60F-254, layer thickness 0.25 mm or liquid chromatography-massspectrometry (LC-MS).

Typically the analytical LC-MS system used consisted of a Waters SQD2platform with electrospray ionization in positive and negative detectionmode with an Acquity UPLC I-class solvent manager, column manager,sample manager and PDA detector. The column for standard method wasCORTECS UPLC C18 1.6 μm, 2.1×30 mm, and the column for polar method wasACQUITY UPLC HSST3 1.8 μm, 2.1×30 mm, the column temperature was 40° C.,the flow rate was 0.7 mL/min, and injection volume was 1 μL. UVdetection was in the range 210-400 nm. The mobile phase consisted ofsolvent A (water plus 0.05% formic acid) and solvent B (acetonitrileplus 0.05% formic acid) with different gradients for 4 differentmethods: 1/Starting with 99% solvent A for 0.2 min changing to 98%solvent B over 1 min, maintained for 0.4 min, then reverting to 99%solvent A over 0.1 min; 2/Starting with 99% solvent A for 0.5 minchanging to 98% solvent B over 3.7 min, maintained for 0.4 min, thenreverting to 99% solvent A over 0.1 min; 3/Starting with 100% solvent Afor 0.4 min changing to 98% solvent B over 0.9 min, maintained for 0.3min, then reverting to 100% solvent A over 0.1 min; 4/Starting with 100%solvent A for 0.8 min changing to 98% solvent B over 3.4 min, maintainedfor 0.4 min, then reverting to 100% solvent A over 0.1 min.

Preparative HPLC purifications were usually performed using a massspectrometry directed system. Usually they were performed on a WatersChromatography Workstation (MassLynx V4.1) configured with LC-MS SystemConsisting of: Waters ZQ™ 2000 (quad MS system with ElectrosprayIonization), Waters 2545 Gradient Pump, Waters 2767 Injecto/Collector,Waters 2998 PDA Detector, the MS Conditions of: 100-1400 amu, PositiveElectrospray, Collection Triggered by MS, and a Waters SUNFIRE® C-18 5micron, 19 mm (id)×150 mm column. The mobile phases consisted ofmixtures of acetonitrile (5-95%) in water containing 0.02% HCOOH. Flowrates were maintained at 20 mL/min, the injection volume was 500 to 3000μL, and the UV detection range was 210-400 nm. Mobile phase gradientswere optimized for the individual compounds. Preparative HPLC were alsoperformed on a Gilson system 233XL WITH 735 (Unipoint). The column was aWaters XBridge Prep C18 5 μm OBD, dimension 30×250 mm. The mobile phaseconsisted of mixture of acetonitrile/ammonium carbonate 0.02N (20-50% in59 min). How rates were maintained at 30 mL/min, the injection volumewas 1000 μL, and the UV detection range was 260 nm. Reactions performedusing microwave irradiation were normally carried out using an EmrysOptimizer manufactured by Personal Chemistry, or an Initiatormanufactured by Biotage. Concentration of solutions was carried out on arotary evaporator under reduced pressure. Flash chromatography wasusually performed using a Biotage® Flash Chromatography apparatus(bolero) on silica gel (15-45μ, 40-63μ, or spheric silica) in pre-packedcartridges of the size noted. ¹H NMR. spectra were acquired at 400 MHzor 500 MHz spectrometers in CDCl₃ solutions unless otherwise noted.Chemical shifts were reported in parts per million (ppm).Tetramethylsilane (TMS) was used as internal reference in CD₃Clsolutions, and residual CH₃OH peak or TMS was used as internal referencein CD₃OD solutions. Coupling constants (J) were reported in hertz (Hz).Chiral analytical chromatography was performed on one of CHIRALPAK® AS,CHIRALPAK® AD, CHIRALCEL® OD, CHIRALCEL® IA, or CHIRALCEL® OJ columns(250×4.6 mm) (Daicel Chemical Industries, Ltd.) with noted percentage ofeither ethanol in hexane (%Et/Hex) or isopropanol in heptane (%IPA/Hep)as isocratic solvent systems. Chiral preparative chromatography wasconducted on one of of CHIRALPAK AS, of CHIRALPAK AD, CHIRALCEL® OD,CHIRALCEL® IA, CHIRALCEL® OJ columns (20×250 mm) (Daicel ChemicalIndustries, Ltd.) with desired isocratic solvent systems identified onchiral analytical chromatography or by supercritical fluid (SFC)conditions.

It is understood that a chiral center in a compound may exist in the “S”or “R” stereoconfiguration, or as a mixture of both. Within a molecule,each bond drawn as a straight line from a chiral center includes boththe (R) and (S) stereoisomers as well as mixtures thereof.

The compounds of Formula I herein including those in Examples 1-52contain a phosphorus chiral center. The isomer mixture in each ofExamples 1-19, were separated, providing an Isomer #A, e.g., Isomer 1A(faster eluting isomer) and an Isomer #B, e.g., Isomer 1B (slowereluting isomer), based on their observed elution order resulting fromthe separation as performed in the Example. Where retention times areshown, they are provided only to show the relative order of elution ofeach isomer in an Example; a retention time does not represent animmutable characteristic of the isomer with which it is associated.Elution order of separated isomers may differ if performed underconditions different than those employed herein. Absolutestereochemistry (R or S) of the phosphorus chiral center in each of the“A” and “B” stereoisomers in Examples 1 to 19 was not determined. Anasterisk (*) may be used in the associated chemical str e drawings ofthe Example compounds to indicate the phosphorus chiral center.

Isopropyl 2-amino-2-methylpropanoate hydrochloride: Thionyl chloride(24.2 g, 204 mmol) was added dropwise to a solution of2-amino-2-methylpropanoic acid (7 g, 67.9 mmol) in propan-2-ol (250 mL)at rt. The mixture was heated at 85° C. (reflux) for 5 days. Theresulting clear solution was concentrated under reduced pressure to givea residue, which was triturated with diethyl ether and pentane to affordthe title compound: ¹H NMR (400 MHz, DMSO-d₆) δ 8.55 (brs, 3H), 4.98(heptuplet, J=6.25 Hz, 1H), 1.45 (s, 6H), 1.44 (s, 1H), 1.24 (d, J=6.25Hz, 6H).

Pentyl 2-amino-2-methylpropanoate hydrochloride: Thionyl chloride (34.6g, 291 mmol) was added dropwise to a solution of2-amino-2-methylpropanoic acid (10 g, 97 mmol) in pentan-1-ol (200 mL)at −50° C. The mixture was allowed to warm to ambient temperature, andthen heated at 80° C. for 16 hours. The resulting mixture wasconcentrated under reduced pressure to give a residue, which wastriturated with ice-cold diethyl ether to afford the title compound:LC/MS: [(M-HCl+1)]⁺=174.0.

Cyclopentyl 2-amino-2methylpropanoate hydrochloride: Thionyl Chloride(3.87 mL, 53.3 mmol) was added dropwise to a stirred solution of2-amino-2-methylpropanoic acid (5 g, 48.5 mmol) in cyclopentanol (26.4ml, 291 mmol) at 0° C. The mixture was stirred at 80° C. for 16 hours.The reaction mixture was cooled, diluted with water (60 mL) and washedwith DCM (2×80 mL). The aqueous layer was concentrated under reducedpressure to afford the title compound: ¹H NMR (400 MHz, dMSO-d₆) δ 8.61(brs, 3H), 5.19-5.15 (m, 1H), 1.87-1.81 (m, 2H) 1.72-1.55 (m, 6H), 1.45(s, 6H).

Isopropyl (S)-2-hydroxypropanoate: Thionyl chloride (5.4 g, 45.4 mmol)was added dropwise to a solution of (S)-2-hydroxypropanoic acid (2 g,22.2 mmol) in propan-2-ol (15 mL) at rt. The mixture was heated at 80°C. under microwaves irradiations for 2 hours. The resulting solution wasconcentrated under reduced pressure to give a residue, which wasdissolved in EtOAc and washed with sat. aqueous sodiumhydrogenocarbonate solution. The combined organic layers were dried,filtered and concentrated under reduced pressure to afford the titlecompound: ¹H NMR (400 MHz, DMSO-d₆) δ 5.28 (d, J=5.73 Hz, 1H), 4.90(heptuplet, J=6.30 Hz, 1H), 4.10-4.04 (m, 1H), 1.23-1.18 (m, 9H).

Cyclopentvl (S),-hvdroxypropanoate: Thionyl chloride (42.3 g, 355 mmol)was added dropwise to a solution of (S)-2-hydroxypropanoic acid (16 g,178 mmol) in cyclopentanol (120 mL) at rt. The reaction mixture washeated at 105° C. for 2 hours and under microwaves irradiations for 45minutes. The resulting solution was concentrated under reduced pressureto give a residue, which was dissolved in EtOAc and washed with sat.aqueous sodium hydrogenocarbonate solution and brine. The combinedorganic layers were dried, filtered and concentrated under reducedpressure. The resulting residue was distilled under high vacuum at 50°C.-100° C. to afford the title compound: ¹H NMR (400 MHz, CDCl₃) δ5.27-5.24 (m, 1H), 4.22 (q, J=6.86 Hz, 1H), 2.80 (brs, 1H), 1.91-1.47(m, 8H), 1.40-1.38 (m, 3H).

Isopropyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(3-cyanophenoxv)phosphoryl)amino)-2-methyluropanoate:To a mixture of (R)-(((1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)phosphonic acid (referred toherein as TFV, 3.5 g, 12.19 mmol), 3-hydroxybenzonitrile (7.26 g, 60.9mmol), INTERMEDIATE A (4.43 g, 24.37 mmol) and triethylamine (14.80 g,146 mmol) in pyridine (42 mL) heated at 60° C. for 5 min was added afreshly prepared solution of 1,2-di(pyridin-2-yl)disulfane (Aldrithiol,18.79 g, 85 mmol) and triphenylphosphine (22.37 g, 85 mmol) in pyridine(42 mL), The reaction mixture was stirred at 60° C. overnight, cooled tort, and concentrated under reduced pressure. The crude residue waspartitioned between EtOAc and an aqueous saturated NaHCO₃ solution. Theorganic layer was washed with brine, dried and concentrated underreduced pressure. The crude residue was purified by flash chromatographyon silica gel (DCM/EtOH: 0 to 20%) to afford the title compound as adiastereomeric mixture: ³¹P NMR (162 MHZ, DMSO-d₆) δ 22.83 (s, 0.6P),22.77 (s, 0.4P); LC/MS: [(M+1)]⁺=516.5.

Pentyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(4-chlorophenoxy)phosphoryl)amino)-2-methylpropanoate:To a mixture of (R)-(((1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)phosphonic acid (referred toherein as TFV, 14.6 g, 50.8 mmol), INTERMEDIATE B (10.6 g, 50.8 mmol),4-chlorophenol (13.1 g, 102 mmol), and triethylamine (51.3 g, 508 mmol)in pyridine (500 mL) were added triphenylphosphine (53.3 g, 203 mmol)and 1,2-di(pyridin-2-yl)disulfane (Aldrithiol, 44.8 g, 203 mmol). Thereaction mixture was stirred for 16 hours at 60° C. under nitrogenatmosphere. After cooling down to ambient temperature, the resultingmixture was concentrated under reduced pressure and the residue waspurified by silica gel column chromatography DCM/MeOH:

2% to 10%) to afford the title compound: LC/MS: [(M+1)]⁺=553.2.

Cyclopentyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(3-cyanophenoxyphosphoryl)amino)-2-methylpropanoate: INTERMEDIATE H was prepared in asimilar fashion to that described for the synthesis of INTERMEDIATE Fstarting from TFV with INTERMEDIATE C: LC/MS: [(M+1)]⁺=542.6.

Hexyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE I wasprepared in a similar fashion to that described for the synthesis ofINTERMEDIATE A starting from 2-amino-2-methylpropanoic acid inhexan-1-ol: ¹H NMR. (400 MHz, DMSO-d₆) δ 8.75 (brs, 3H), 4.15 (t, J=6.48Hz, 2H), 1.65-1.57 (m, 2H), 1.49 (s, 6H), 1.37-1.25 (m, 6H), 0.88-0.85(m, 3H).

Butyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE J wasprepared in a similar fashion to that described for the synthesis ofINTERMEDIATE A starting from 2-amino-2-methylpropanoic acid in1-butanol: ¹H NMR (400 Hz, DMSO-d₆) δ 8.63 (brs, 3H), 4.17 (t, J=6.44Hz, 2H), 1.64-1.57 (m, 2H), 1.48 (s, 6H), 1.44-1.31 (m, 2H), 0.9 (t,J=7.39Hz, 3H).

Propyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE K wasprepared in a similar fashion to that described for the synthesis of:INTERMEDIATE A starting from 2-amino-2-methylpropanoic acid inpropan-1-ol: ¹H NMR (400 MHz, DMSO-d₆) δ 8.66 (brs, 3H), 4.13 (t, J=6.44Hz, 2H), 1.68-1.59 (m, 2H), 1.49 (s, 6H), 0.91 (t, J=7.49 Hz, 3H).

Ethyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE L wasprepared in a similar fashion to that described for the synthesis ofINTERMEDIATE C starting from 2-amino-2-methylpropanoic acid in ethanoland worked-up in a similar fashion to that described for INTERMEDIATE A:¹H NMR (400 MHz, DMSO-d₆) δ 8.75 (brs, 3H), 4.20 (q, J=7.06 Hz, 2H),1.49 (s, 6H), 1.23 (t, J=7.06 Hz, 3H).

Isobutyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE M wasprepared in a similar fashion to that described for the synthesis ofINTERMEDIATE A starting from 2-amino-2-methylpropanoic acid inisobutanol: ¹NMR (400 MHz, DMSO-d₆) δ 8.69 (brs, 3H), 3.96 (d, J=6.37Hz, 2H), 1.93 (heptuplet, J=6.62 Hz, 1H), 1.50 (s, 6H), 0.92 (d, J=6.62Hz, 6H).

Cyclohexyl 2-amino-2-methylpropanoate hydrochloride: INTERMEDIATE N wasprepared in a similar fashion to that described for the synthesis ofINTERMEDIATE A starting from 2-amino-2-methylpropanoic acid incyclohexanol (the residue was triturated twice): ¹H NMR (400 MHz,DMSO-d₆) δ 8.47 (brs, 3H), 4.83-4.78 (m, 1H), 1.80-1.74 (m, 2H),1.70-1.63 (m, 2H), 1.53-1.29 (m, 12H).

Cyclobutyl 2-amino-2-methylpropanoate hydrochloride: To a stirredsolution of cyclobutanol (5.03 g, 69.8 mmol) at rt thionyl chloride(1.52 g, 53.3 mmol) was added dropwise, followed by2-amino-2-methylpropanoic acid (1.2 g, 11.64 mmol). The reaction mixturewas capped and stirred at 80° C. for 2 days. The reaction mixture wascooled, partitioned between DCM (40 mL) and water (30 mL). The aqueouslayer was washed with DCM (40 mL), concentrated azeotroping with ACN,and dried under high pressure. The resulting solid was washed withdiethyl ether to afford the title compound: ¹H NMR (400 MHz, dMSO-d₆) δ8.70 (brs, 3H), 4.99 (quintuplet, J=7.38 Hz, 1H), 2.35-2.27 (m, 2H),2.12-2.02 (m, 2H), 1.83-1.74 (m, 1H), 1.69-1.57 (m, 1H), 1.48 (s, 6H).

Benzyl 2-amino-2-methylpropanoate hydrochloride: A mixture of2-amino-2-methylpropanoic acid (3 g, 29.1 mmol), benzyl alcohol (12 mL,115 mmol) and p -toluenesulfonic acid monohydrate (5.53 g, 29.1 mmol) intoluene (40 mL) was heated under reflux overnight. Additional benzylalcohol (12 mL, 115 mmol) was added and the reaction mixture was heatedwith a Dean Starck overnight. The reaction mixture was concentratedunder reduced pressure and washed with diethyl ether. Diethyl ether wasremoved, and then the residue was dissolved in ethyl acetate and washedwith a saturated aqueous NaHCO₃ solution. The organic layer was driedover Na₂SO₄ and concentrated under reduced pressure. The resulting oilwas triturated with diethyl ether and HCl in dioxane. The resultingsolid was filtered, washed with diethyl ether and dried under reducedpressure to afford the title compound: ¹H NMR (400 MHz, dMSO-d₆) δ 8.54(brs, 3H), 7.42-7.35 (m, 5H), 5.25 (s, 2H), 1.49 (s, 6H).

Isopropyl (R)-2-hydroxypropanoate: INTERMEDIATE Q was prepared in asimilar fashion to that described for the synthesis of INTERMEDIATE Dstarting from (R)-2-hydroxypropanoic acid in propan-2-ol: ¹H NMR (400MHz, DMSO-d₆) δ 5.27 (d, J=5.82 Hz, 1H), 4.91 (heptuplet, J=6.22 Hz,1H), 4.11-4.01 (m, 1H), 1.24-1.18 (m, 9H).

Isopropyl (S)-2-hydroxy-3-phertyJpropanoate: INTERMEDIATE R was preparedin a similar fashion to that described for the synthesis of INTERMEDIATEstarting from (S) -2-hydroxy-3-phenylpropanoic acid in propan-2-ol: ¹HNMR (400 MHz, DMSO-d₆) δ 7.28-7.17 (m, 5H), 5.48 (d, J=6.15 Hz, 1H),4.86 (heptuplet, J=6.21 Hz, 1H), 4.21-4.16 (m, 1H), 2.92 (dd, J=13.64Hz, 5.65 Hz, 1H), 2.83 (dd, J=13.64 Hz, 7.71 Hz, 1H), 1.16 (d, J=6.21Hz, 3H), 1.08 (d, J=6.21 Hz, 3H).

Cyclopentyl 2-hydroxyacetate: INTERMEDIATE S was prepared in a similarfashion to that described for the synthesis of INTERMEDIATE D startingfrom glycolic acid in cyclopentanol. The reaction was cartied out at100° C. The obtained oil was distilled to afford the title compound: ¹HNMR (400 MHz, CDCl₃) δ 5.30-5.26 (m, 1H), 4.11 (s, 2H), 2.80 (s, 1H),1.95-1.84 (m, 2H), 1.77-1.68 (m, 4H), 1.64-1.56 (m, 2H).

Isopropyl 2-hydroxy-2-methylpropanoate: INTERMEDIATE T was prepared in asimilar fashion to that described for the synthesis of INTERMEDIATE Dstarting from 2-hydroxy -2-methylpropanoic acid in propan-2-ol: ¹H NMR(400 MHz, DMSO-d₆) δ 5.14 (s, 1H), 4.88 (heptuplet, J=6.27 Hz, 1H), 1.27(s, 6H), 1.19 (d, J=6.27 Hz, 6H).

Pentyl (S)-2-hydroxypropanoate: Step 1: To a solution of(S)-2-hydroxypropanoic acid (0.90 g, 9.99 mmol) in THF (20 mL) wereadded tert-butylchlorodimethylsilane (3.31 g, 21.98 mmol) and imidazole(1.63 g, 23.98 mmol) at 0° C. The resulting mixture was stirred for 4hours at ambient temperature and diluted with water (50 mL). Theresulting mixture was acidified with HCl (24 mL, 1 M in water),extracted with EtOAc (3×40 mL) and washed with brine (50 mL). Thecombined organic layers was dried over anhydrous sodium sulfate andfiltered. The filtrate was concentrated under reduced pressure to affordthe title intermediate: ¹H NMR (300 MHz, CDCl₃) δ 4.37 (q, J=6.9 Hz,1H), 1.45 (d, J=6.9 Hz, 3H), 0.93 (s, 9H), 0.15 (s, 6H). Step 2: To asolution of (S)-2-((tert -butyldimethylsilyl)oxy)propanoic acid (0.50 g,2.45 mmol) in DCM (20 mL) was added N,N′-carbonyldiimidazole (0.59 g,3.67 mmol) followed by the addition of pentan-1-ol (0.32 g, 3.67 mmol).The resulting mixture was stirred for 16 hours at ambient temperature.The reaction was quenched with water (100 mL) and the resulting mixturewas extracted with EtOAc (3×50 mL). The combined organic layers waswashed with brine (2×50 mL), dried over anhydrous sodium sulfate andfiltered. The filtrate was concentrated under reduced pressure and theresidue was purified by silica gel column chromatography (PE/EtOAc: 3%to 10%) to afford the title intermediate: ¹H NMR (300 MHz, CDCl₃) δ 4.32(q, J=6.9 Hz, 1H), 4.15-4.06 (m, 2H), 1.68-1.64 (m, 2H), 1.39 (d, J=6.6Hz, 3H), 1.36-1.25 (m, 4H), 0.92-0.89 (m, 3H), 0.89 (s, 9H), 0.12 (s,3H), 0.09 (s, 3H). Step 3: A solution of (S)-pentyl2-((tert-butyldimethylsilyl)oxy)propanoate (1.16 g, 4.24 mmol) in THE(20 mL) was treated. with tetrabutylarnmonium fluoride (1.66 g, 6.35mmol) for 2 hours at ambient temperature. The reaction was quenched withwater (100 mL) and the resulting mixture was extracted with EtOAc (3×50mL). The combined organic layers was washed with brine (2×50 mL), driedover anhydrous sodium sulfate and filtered. The filtrate wasconcentrated under reduced pressure and the residue was purified bysilica gel column chromatography (PE/EtOAc: 3% to 10%) to afford thetitle compound: ¹H NMR (300 MHz, CDCl₃) δ 4.30 (q, J=6.9 Hz, 1H),4.19-4.15 (m, 2H), 2.80 (d, J=4.2 Hz, 1H), 1.69-1.65 (m, 2H), 1.39 (d,J=7.2 Hz, 3H), 1.35-1.29 (m, 4H), 0.92-0.89 (m, 3H).

Isopropyl 1-aminocyclopentane-1-carboxylate hydrochloride: INTERMEDIATEV was prepared in a similar fashion to that described for the synthesisof INTERMEDIATE D starting from cycloleucine in propan-2-ol. The cruderesidue was triturated in PE/diisopropyl ether mixture to afford thetitle compound: ¹H NMR (400 MHz, DMSO-d₆) δ 8.61 (brs, 3H), 4.98(heptuplet, J=6.26 Hz, 1H), 2.13-2.05 (m, 2H), 1.93-1.70 (m, 6H), 1.24(d, J=6.26 Hz, 6H).

Isopropyl 1-aminocyclopropane-1-carboxylate hydrochloride: Thionylchloride (3.26 g, 27.4 mmol) was added dropwise to a solution of ethyl1-aminocyclopropane-1-carboxylate (2 g, 11.8 mmol) in propan-2-ol (8 mL)at 0° C. The mixture was heated at 110° C. for 16 hours. The resultingsolution was concentrated under reduced pressure to afford the titlecompound: ¹H NMR (400 MHz, DMSO-d₆) δ 9.01-8.87 (m, 3H), 4.95(heptuplet, J=6.19 Hz, 1H), 1.47-1.33 (m, 4H), 1.21 (d, J=6.19Hz, 6H).

Isopropyl 1-aminocyclobutane-1-carboxylate hydrochloride: INTERMEDIATE Xwas prepared in a similar fashion to that described for the synthesis ofINTERMEDIATE A starting from 1-aminocyclobutanecarboxylic acid inpropan-2-ol for 16 hours. The crude residue was triturated in diethylether to afford the title compound: ¹H NMR (400 MHz, DMSO-d₆) δ 8.88(brs, 3H), 5.02 (heptuplet, J=6.22 Hz, 1H), 2.48-2.41 (m, 4H), 2.10-1.97(m, 2H), 1.28 (d, J=6.22 Hz, 6H).

Isopropyl 1-aminocyclohexane-1-carboxylate hydrochloride: Thionylchloride (14.93 g, 125 mmol) was added dropwise to a solution of1-aminocyclohexanecarboxylic acid (8 g, 55.9 mmol) in propan-2-ol (250mL). The reaction mixture was heated at 85° C. for 16 hours. Thionylchloride (7.19 g, 60.5 mmol) was added and the mixture was heated at 90°C. for additional 16 hours. Thionyl chloride (4.91 g, 41.2. mmol) wasadded and the mixture was heated at 100° C. for additional 16 hours. Thereaction mixture was concentrated under reduce to give a residue whichwas triturated with diethyl ether to afford the title compound: ¹H NMR(400 MHz, DMSO-d₆) δ 8.58 (brs, 3H), 4.99 (heptuplet, J=6.21 Hz, 1H),1.97-1.90 (m, 2H), 1.78-1.72 (m, 2H), 1.69-1.59 (m, 2H), 1.58-1.49 (m,2H), 1.45-1.37 (m, 2H), 1.25 (d, J=6.21 Hz, 6H).

EXAMPLE 1

Step 1: Isopropyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate:To a stirred mixture of INTERMFDIATE F (3.5 g, 6.79 mmol) andINTERMEDIATE D (1.79 g, 13.58 mmol) in THF (20 mL) was added DBU (1.53mL, 10.18 mmol) at rt. The reaction mixture was stirred at RT for 3hours and then, concentrated under reduced pressure. The crude residuewas purified twice by flash chromatography on silica gel (DCM/MeOH: 0 to10%) to afford the title compound as a mixture of diastereoisomers: ³¹PNMR (162 MHz, CDCl₃) d 24.78 (s, 0.4P), 24.60 (s, 0.6P); LC/MS:[(M+1)]⁺=529.2.

Step 2: Isopropyl2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate;and isopropyl2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxyopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate(1A and 1B):

The isomers were separated by preparative chiral SFC with the followingconditions: Column: ID 2*25 cm; Mobile Phase A: CO₂: 70%, Mobile PhaseB: IPA (HPLC grade): 30%; Flow rate: 60 mL/min; Detector 220 nm; toafford: Isomer IA (faster eluting): ¹H NMR (600 MHz, CDCl₃) δ 8.33 (s,1H), 8.10 (s, 1H), 5.98 (brs, 2H), 5.11 (p, J=6.3 Hz, 1H), 5.05-4.94 (m,2H), 4.39 (dd, J=14.5 Hz, 2.9 Hz, 1H), 4.16 (dd, J=14.4 Hz, 7.5 Hz, 1H),3.94 (ddd, J=7.4 Hz, 6.1 Hz, 2.9 Hz, 1H), 3.90-3.80 (m, 2H), 3.60 (dd,J=13.1 Hz, 9.7 Hz, 1H), 1.53 (d, J=7.0 Hz, 3H), 1.45 (s, 3H), 1.43 (s,3H), 1.31-1.19 (m, 15H); ³¹P NMR (243 MHz; CDCl₃) δ 24.687; LC/MS[(M+1)]⁺=529.4; and Isomer 1B (slower eluting).

Isomer 1B was then re-purified by preparative SFC with the followingconditions: Column: AD-H 2*25 cm; Mobile Phase A: CO₂: 80%, Mobile PhaseB: IPA (HPLC grade): 20%; Flow rate: 60 ml/min; Detector 220 nm: ¹NMR(600 MHz, CDCl₃) d 8.34 (s, 1H), 8.11 (s, 1H), 5.80 (s, 2H), 5.09-4.87(m, 3H), 4.41 (dd, J=14.6 Hz, 3.0 Hz, 1H), 4.19 (dd, J=14.5 Hz, 6.9 Hz,1H), 4.04-3.97 (m, 2H), 3.87 (dd, J=13.0 Hz, 6.9 Hz, 1H), 3.79 (dd,J=13.1 Hz, 11.6 Hz, 1H), 1.54 (bs, 6H) 1.44 (d, J=7.0 Hz, 3H), 1.28-1.19(m, 15H); ³¹P NMR (243 MHz, CDCl₃) d 24.828 (s, 1P); LC/MS:[(M+1)]⁺=529.3.

EXAMPLE 2

Step 1: Pentyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)(phosphoryl)amino)-2-methylpropanoate:To a solution of INTERMEDIATE G (7.0 g, 12.6 mmol) and INTERMEDIATE D(2.5 g, 18.9 mmol) in tetrahydrofuran (50 mL) was added DBU (1.9 g, 12.7mmol). The reaction mixture was stirred at rt for 16 hours. Then theresulting mixture was concentrated under reduced pressure and the cruderesidue was purified by silica gel column chromatography (DCM/MeOH: 2%to 10%) to afford the title compound as a mixture of diastereoisomers.

Step 2: Pentyl2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate and

Pentyl 2-(((R)-((((R-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxv-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate(2A and 2B): The two isomers were separated by chiral preparative SFCwith the following conditions: Column: Chiralpak IA 2*25 cm, 20 μm;Mobile Phase A: CO₂: 70%, Mobile Phase B: IPA (HPLC grade): 30%; Flowrate: 150 mL/min; Detector 254 nm; to afford Isomer 2A (faster eluting):¹H NMR (400 MHz; CD₃OD) δ 8.22 (s, 2H), 5.11-5.09 (m, 1H), 5.08-4.99 (m,1H), 4.41 (dd, J=14.8, 2.8 Hz, 1H), 4.28-4.22 (m, 1H), 4.11 (t, J=6.8Hz, 2H), 4.03-3.99 (m, 1H), 3.95-3.89 (m, 1H), 3.72-3.66 (m, 1H),1.66-1.62 (m, 2H), 1.48-1.43 (m, 9H), 1.39-1.31 (m, 4H), 1.28 (d, J=7.6Hz, 6H), 1.24 (d, J=6.4 Hz, 3H), 0.94 (t, J=6.8 Hz, 3H); ³¹P NMR (162MHz; CD₃OD) δ 26.39; LC/MS: [(M+1)]⁺=557.3; and Isomer 2B (slowereluting): ¹H NMR (400 MHz; CD₃OD). δ 8.26 (s, 1H), 8.23 (s, 1H),5.03-5.01 (m, 1H), 4.99-4.97 (m, 1H), 4.42 (dd, J=14.0, 2.8 Hz, 1H),4.29-4.24 (m, 1H), 4.14 (t, J=6.8 Hz, 2H), 4.08-4.02 (m, 1H), 3.95-3.90(m, 1H), 3.82-3.76 (m, 1H), 1.71-1.66 (m, 2H), 1.50 (s, 6H), 1.44 (d,J=6.8 Hz, 3H), 1.39-1.31 (m, 4H), 1.29-1.20 (m, 9H), 0.95 (t, J=6.8 Hz,3H); ³¹P NMR (162 MHz; CD₃OD) δ 26.48; LC/MS: [(M+1)]⁺=557.3.

EXAMPLE 3

Step 1: Isopropyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yloxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphonyl)amino)-2-methylpropanoate:To a stirred mixture of TFV (3 g, 10.44 mmol), INTERMEDIATE A (4.22g,20.89 mmol), isopropylglycolate (6.43g, 52.23 mmol) and triethylamine(12.68 g, 125.34 mmol) in pyridine (35 mL) heated at 60° C. for 5 minwas added a freshly prepared solution of 1,2-di(pyridin-2-yl)disulfane(Aldrithiol, 16.67g, 74.16 mmol) and triphenylphosphine (19.65 g, 74.16mmol) in pyridine (35 mL). The reaction mixture was stirred at 60° C.overnight, cooled to rt, and concentrated under reduced pressure. Thecrude residue was partitioned between EtOAc and an aqueous saturatedNaHCO₃ solution. The organic layer was washed with brine, dried andconcentrated under reduced pressure. The crude residue was purified byflash chromatography on silica gel (DCM/MeOH) to afford the titlecompound as a diastereomeric mixture.

Step 2: Isopropyl2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoateand Isopropyl2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate (3A and 3B): The twoisomers were separated by chiral Preparative HPLC with the followingconditions: Column: Chiralpak IC 0.46*25 cm. 5 μm; Mobile Phase A: CO₂:60%, Mobile Phase B: IPA: 40%; to afford Isomer 3A (faster eluting): ¹HNMR (400 MHz; CDCl₃) δ 8.35 (s, 1H), 8.12 (s, 1H), 5.72 (s, 2H), 5.08(heptuplet, J=6.21 Hz, 1H), 5.04 (hept, J=6.21 Hz, 1H), 4.56 (dd,J=16.32 Hz and 11.88 Hz, 1H), 4.46 (dd, J=16.32 Hz and 12.06 Hz, 1H),4.40 (dd, J=14.37 Hz and 3.02 Hz, 1H), 4.19 (dd, J=14.37 Hz and 7.09 Hz,1H); 4.03-3.96 (m, 2H), 3.88 (dd, J=13.30 Hz and 7.45 Hz, 1H), 3.72 (dd,J=13.30 Hz and 10.11 Hz, 1H), 1.52 (d, J=3.87 Hz, 6H), 1.27-1.25 (m,12H), 1.22 (d, J=6.21 Hz, 3H); ³¹P NMR (198 MHz; CDCl₃) δ 26.06; LC/MS:[(M+1)]⁺=515.4; and Isomer 3B (slower eluting): ¹H NMR (400 MHz; CDCl₃).δ 8.35 (s, 1H), 8.11 (s, 1H), 5.71 (s, 2H), 5.12 (heptuplet, J=6.25 Hz,1H), 4.99 (hept, J=6.25 Hz, 1H), 4.67 (dd, J=16.32 Hz and 11.83 Hz, 1H),4.45 (dd, J=16.32 Hz and 12.43 Hz, 1H), 4.39 (dd, J=14.38 Hz and 2.38Hz, 1H), 4.16 (dd, J=14.38 Hz and 7.49 Hz, 1H); 3.99-3.86 (m, 3H), 3.63(dd, J=13.29 Hz and 9.44 Hz, 1H), 1.45 (d, J=4.56 Hz, 6H), 1.29-1.22 (m,15H), ³¹P NMR (198 MHz; CDCl₃) δ 25.90; LC/MS: [(M+1)]⁺=515.7;

EXAMPLE 4

Step 1: Pentyl2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate:Compounds 4A and 4B were prepared in a similar fashion to that describedfor the synthesis of Compounds 2A and 2B starting from INTERMEDIATE Gwith isopropylglycolate.

Step 2: Pentyl2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoateand Pentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate(4A and 4B): The two isomers were separated by chiral preparative SFCwith the following condition: Column: Chiralpak IA 2*25 cm, 5 μm; MobilePhase A: CO₂: 75%, Mobile Phase B: IPA: 25%; Flow rate: 40 mL/min;Detector: 254 nm; to afford Isomer 4A (faster eluting): ¹H NMR (400 MHz;CD₃OD) δ 8.25 (s, 1H), 8.23 (s, 1H), 5.12-5.06 (m, 1H), 4.62 (dd,J=16.4, 10.4 Hz, 1H), 4.49 (dd, J=16.0, 10.4 Hz, 1H), 4.42 (dd, J=14.4,2.8 Hz, 1H), 4.24 (dd, J=14.8, 7.6 Hz, 1H), 4.11 (t, J=6.8 Hz, 2H),4.03-3.94 (m, 2H), 3.72 (dd, J=13.2, 9.6 Hz, 1H), 1.67-1.64 (m, 2H),1.46 (s, 3H), 1.44 (s, 3H), 1.38-1.30 (m, 4H), 1.29-1.25 (m, 6H), 1.24(d, J=6.0 Hz, 3H), 0.94 (t, J=6.8 Hz, 3H); ³¹P NMR (162 MHz; CD₃OD) δ27.57; LC/MS: [(M+1)]⁺=543.2; and Isomer 4B (slower eluting): ¹H NMR(400 MHz; CD₃OD) δ 8.24 (s, 1H), 8.22 (s, 1H), 5.10-5.03 (m, 1H), 4.51(d, J=10.8 Hz, 2H), 4.41 (dd, J=14.4, 2.8 Hz, 1H), 4.26 (dd, J=14.4, 7.2Hz, 1H), 4.13 (t, J=6.8 Hz, 2H), 4.06-4.01 (m, 1H), 3.93 (dd, J=16.0,8.0 Hz, 1H), 3.78 (dd, J=13.4, 9.6 Hz, 1H), 1.69-1.64 (m, 2H), 1.49 (s,3H), 1.48 (s, 3H), 1.39-1.32 (m, 4H), 1.29-1.25 (m, 6H), 1.22 (d, J=6.0Hz, 3H), 0.94 (t, J=6.8 Hz, 3H); ³¹P NMR (162 MHz; CD₃OD) δ 7.67; LC/MS:[(M+1)]⁺=543.2.

The compounds in the following Examples were prepared in an analogousfashion to that described for Examples 1-4, wherein Step 1 describespreparation of the compound as a mixture of diastereoisomers. Thediastereoisomer mixtures were separated in Examples 1-19 and were notseparated for compounds in Examples 20-52. Conditions for theseparations are provided in Examples 1-19. Isomers were separated byeither preparative HPLC, preparative chiral HPLC or preparative chiralSFC. The noted “Intermediates” in the following Examples refer to theintermediate compound name or intermediate example letters (e.g.,Intermediates C and E; or Intermediates: J and Isopropyl glycolate) thatdescribe each of the intermediates that were used in Step 1 for eachExample.

Example 5 Cyclopentyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9-yl)propan- 2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan- 2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; and Cyclopentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl) oxy)methyl)(((S)-1-(cylopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl) amino)-2-methylpropanoateIntermediates: C and E Isomer separation/purification conditions:Preparative chiral SFC: CHIRALPAK-AD-H, 20 * 250 atm; Mobile Phase A:25% EtOH.; Mobile Phase B: CO₂; Flow rate: 70 mL/min; Gradient:isocratic; Detector: UV 220 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺Elution order 5A 24.67 (CDCl₃) 581.7 Faster eluting 5B 24.85 (CDCl₃)581.7 Slower eluting

EXAMPLE 6 Hexyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9-yl)propan- 2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2- methylpropanoate and Hexyl2-(((R)-((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoateIntermediates: I and Isopropyl glycolate Isomer separation/purificationconditions: Preparative chiral HPLC: Chiralpak IA 20 × 250 mm, 5 μm;Mobile Phase A: Hexanes; Mobile Phase B: 30% EtOH; Flow rate: 20 mL/min;Gradient: isocratic; Detector: UV 254/220 nm Isomer ³¹P NMR (ppm) LC/MS(M + 1)⁺ Elution order 6A 27.57 (MeOD) 557.5 Faster eluting 6B 27.66(MeOD) 557.5 Slower eluting

EXAMPLE 7 Hexyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9-yl)propan- 2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2- yl)oxy)phosphoryl)amino)- 2-methylpropanoate,and Hexyl 2-(((R)-((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S))-1-isopropoxy-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate Intermediates: I and D Isomerseparation/purification conditions: Preparative chiral HPLC: ChiralpakIA 20 × 250 mm, 5 μm; Mobile Phase A: Hexanes; Mobile Phase B: 30% EtOH;Flow rate: 20 mL/min; Gradient: isocratic; Detector: UV 254/220 nmIsomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 7A 26.40 (MeOD) 571.4Faster eluting 7B 26.52 (MeOD) 571.4 Slower eluting

EXAMPLE 8 Isopropyl 2-(((R)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(2-oxo-2-(pentyloxy)ethoxy) phosphoryl)amino)-2- methylpropanoate andIsopropyl 2-(((S)-((((R)-1- (6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl) (2-oxo-2-(pentyloxy)ethoxy)phosphoryl)amino)-2- methylpropanoate Intermediates: A and Pentylglycolate Isomer separation/purification conditions: Preparative chiralHPLC: Chiralpak IA 20 × 250 mm, 5 μm; Mobile Phase A: Hexanes; MobilePhase B: 30% EtOH; Flow rate: 16 mL/min; Gradient: isocratic; Detector:UV 254/220 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 8A 27.51(MeOD) 543.3 Faster eluting 8B 27.60 (MeOD) 543.3 Slower eluting

EXAMPLE 9 Hexyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy) methyl)(((R)-1- isopropoxy-1-oxopropan-2- yl)oxy)phosphoryl) amino)-2- methylpropanoate, and Hexyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((R)-1-isopropoxy- 1-oxopropan-2-yl)oxy) phosphoryl)amino)-2-methylpropanoate Intermediates: I and Q Isomer separation/purificationconditions: Preparative HPLC - Waters XBridge Prep C18 5 μm OBD, 30 ×250 mm; Mobile phase A: 0.5N ammonium carbonate/acetonitrile; Mobilephase B: 0.5N ammonium carbonate/water; Gradient: 20-50% A over 59; Flowrate: 30 mL/min; Detector 260 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺Retention time (min) 9A 24.60 (CDCl₃) 571.4 52.30 9B 24.72 (CDCl₃) 571.455.05

EXAMPLE 10 Isopropyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-oxo-1-(pentyloxy)propan-2- yl)oxy)phosphoryl)amino)-2-methylpropanoate and Isopropyl 2-(((R)-((((R)-1- (6-amino-9H-purin-9-yl)propan-2-yl)oxy) methyl)(((S)-1-oxo-1- (pentyloxy)propan-2-yl)oxy)phosphoryl)amino)- 2-methylpropanoate Intermediates: A and U Isomerseparation/purification conditions: Preparative chiral HPLC:CHIRALPAK-AD-H- SL001 20 × 250 mm; Mobile Phase A: Hexanes; Mobile PhaseB: 30% EtOH; Flow rate: 19 mL/min; Gradient: isocratic; Detector: UV254/220 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 10A 26.39(MeOD) 557.2 Faster eluting 10B 26.50 (MeOD) 557.2 Slower eluting

EXAMPLE 11 Butyl 2-(((S)-((((R)-1-(6-

amino-9H-purin-9-yl)propan- 2-yl)oxy)methyl)(((S)-1- (cyclopentyloxy)-1-oxopropan-2-yl)oxy) phosphoryl)amino)-2- methylpropanoate and Butyl2-(((R)-((((R)-1-(6- amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- (cyclopentyloxy)-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate Intermediates: J and E Isomerseparation/purification conditions: Preparative chiral SFC:CHIRALPAK-AD-H, 20 * 250 mm; Mobile Phase A: 15% MeOH/0.1% NH₄OH.;Mobile Phase B CO₂; Flow rate: 80 mL/min; Gradient: isocratic; Detector:UV 220 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 11A 24.68(d₆-DMSO) 569.8 Faster eluting 11B 24.82 (d₆-DMSO) 569.8 Slower eluting

EXAMPLE 12 Cyclohexyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl) amino)-2-methylpropanoate and Cyclohexyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate Intermediates: N and D Isomerseparation/purification conditions: Preparative HPLC - Waters XBridgePrep C18 5 μm OBD, 30 × 250 mm; Mobile phase A: 0.5N ammoniumcarbonate/acetonitrile; Mobile phase B: 0.5N ammonium carbonate/water;Flow rate: 30 mL/min; Gradient 20-50% A over 59 min; Detector 260 nmIsomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Retention time (min) 12A 24.78(CDCl₃) 569.7 39.0 12B 24.62 (CDCl₃) 569.7 40.0

EXAMPLE 13 Isopropyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate and Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate Intermediates: A and E Isomerseparation/purification conditions: Preparative chiral SFC: CHIRALPAKIA-SFC-02 50 mm × 250 mm; Mobile Phase A: 40% MeOH (0.2% DEA); MobilePhase B: CO₂; Flow rate: 160 mL/min; Gradient: isocratic; Detector: 254nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 13A 26.35 (CDCl₃)555.4 Faster eluting 13B 26.43 (CDCl₃) 555.4 Slower eluting

EXAMPLE 14 Cyclopentyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (2-isopropoxy-2-oxoethoxy)phosphoryl)amino)- 2-methylpropanoate and Cyclopentyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoateIntermediates: C and Isopropyl glycolate Isomer separation/purificationconditions: Preparative HPLC - Water XBridge Prep C18 5 μm OBD, 30 × 250mm; Mobile phase A: 0.5N ammonium carbonate/acetonitrile; Mobile phaseB: 0.5N ammonium carbonate/water; Flow rate: 30 mL/min; Gradient: 20-50%A over 59 min; Detector 260 nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺Retention time (min) 14A 25.95 (CDCl₃) 541.7 28.8 14B 26.08 (CDCl₃)541.7 29.9

EXAMPLE 15 Cyclohexyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (2-isopropoxy-2-oxoethoxy)phosphoryl)amino)- 2-methylpropanoate and Cyclohexyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoateIntermediates: N and Isopropyl glycolate Isomer separation/purificationconditions: Preparative HPLC - Waters XBridge Prep C18 5 μm OBD, 30 ×250 mm; Mobile phase A: 0.5N ammonium carbonate/acetonitrile; Mobilephase B: 0.5N ammonium carbonate/water; Flow rate: 30 mL/min; Gradient:20-50% A over 59 min; Detector 260 nm Isomer ³¹P NMR (ppm) LC/MS (M +1)⁺ Retention time (min) 15A 25.94 (CDCl₃) 555.6 35.0 15B 26.06 (CDCl₃)555.7 36.5

EXAMPLE 16 Cyclobutyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (((S)-1-isopropxy-1-oxopropan-2-yl)oxy)phosphoryl) amino)-2-methylpropanoate and Cyclobutyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate Intermediates: O and D Isomerseparation/purification conditions: Preparative HPLC - Waters XBridgePrep C18 5 μm OBD, 30 × 250 mm; Mobile phase A: 0.5N ammoniumcarbonate/acetonitrile; Mobile phase B: 0.5N ammonium carbonate/water;Flow rate: 30 mL/min; Gradient: 20-50% A over 59 min; Detector 260 nmIsomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Retention time (min) 16A 24.86(CDCl₃) 541.2 26.5 16B 24.64 (CDCl₃) 541.2 27.5

EXAMPLE 17 Cyclobutyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (2-isopropxy-2-oxoethoxy)phosphoryl)amino)- 2-methylpropanoate and Cyclobutyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2 -methylpropanoateIntermediates: O and Isopropyl glycolate Isomer separation/purificationconditions: Preparative HPLC - Waters XBridge Prep C18 5 μm OBD, 30 ×250 mm; Mobile phase A: 0.5N ammonium carbonate/Acetonitrile; Mobilephase B: 0.5N ammonium carbonate/water; Flow rate: 30 mL/min; Gradient:20-50% A over 59 min; Detector 260 nm Isomer ³¹P NMR (ppm) LC/MS (M +1)⁺ Retention time (min) 17A 25.96 (CDCl₃) 527.2 22.5 17B 26.09 (CDCl₃)527.2 23.5

EXAMPLE 18 Cyclopentyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl) (((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl) amino)-2-methylpropanoate and Cyclopentyl2-(((R)-((((R)-1- (6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1- oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate Intermediates: C and D Isomerseparation/purification conditions: Preparative chiral SFC:CHIRALPAK-AD-H, 30 * 250 mm; Mobile Phase A: 15% MeOH.; Mobile Phase B:CO₂; Flow rate: 70 mL/min; Gradient: isocratic; Detector: UV 220 nmIsomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 18A 24.72 (d₆-DMSO)555.6 Faster eluting 18B 24.83 (d₆-DMSO) 555.6 Slower eluting

EXAMPLE 19 Pentyl 2-(((S)-((((R)-1-

(6-amino-9H-purin-9- yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2- yl)oxy)phosphoryl)amino)-2-methylpropanoate and Pentyl 2-(((R)-((((R)-1- (6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- (cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate Intermediates: B and EIsomer separation/purification conditions: Preparative chiral SFC:CHIRALPAK AD-H SFC 50 * 250 mm, 5 μm; Mobile Phase A: 40% EtOH; MobilePhase B: CO₂; Flow rate: 160 mL/min; Gradient: isocratic; Detector: 254nm Isomer ³¹P NMR (ppm) LC/MS (M + 1)⁺ Elution order 19A 26.36 (meOD)583.3 Faster eluting 19B 26.47 (MeOD) 583.3 Slower eluting

LC/MS Ex. Structure Name (M + 1)⁺ 20

Isopropyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 529.5 21

Propyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2-methylpropanoate 515.5 22

Propyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 529.4 23

Propyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 529.2 24

Butyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2-methylpropanoate 529.2 25

Butyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 543.2 26

Butyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 543.2 27

Ethyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2-methylpropanoate 501.2 28

Ethyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 515.2 20

Ethyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 515.2 30

Isobutyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2-methylpropanoate 529.6 31

Isobutyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 543.7 32

Isopropyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxo-3- phenylpropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 605.4 33

Hexyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxo-3- phenylpropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 647.9 34

Hexyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- (cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 597.9 35

Hexyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2- yl)oxy)methyl)(2-(cyclohexyloxy)-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoate 583.836

Pentyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2- yl)oxy)methyl)(2-(cyclopentyloxy)-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoate569.7 37

Isobutyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- (cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 569.8 38

Propyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2- yl)oxy)methyl)(2-(cyclopentyloxy)-2- oxoethoxy)phosphoryl)amino)- 2-methylpropanoate541.2 39

Cyclopentyl 2-((((((R)-1-(6- amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 555.5 40

Isopropyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-ethoxy-1- oxopropan-2- yl)oxy)phosphoryl)amino)-2-methylpropanoate 515.5 41

Benzyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 577.7 42

Benzyl 2-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)-2-methylpropanoate 563.6 43

Hexyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-ethoxy-1- oxopropan-2- yl)oxy)phosphoryl)amino)-2-methylpropanoate 557.5 44

Hexyl 2-((((((R)-1-(6-amino-9H- purin-9-yl)propan-2-yl)oxy)methyl)((1-isopropoxy-2- methyl-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2- methylpropanoate 585.4 45

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)cyclopentane-1-carboxylate 541.7 46

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)cyclopropane-1-carboxylate 513.4 47

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino) cyclopropane-1-carboxylate 527.5 48

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclo- pentane-1-carboxylate 555.4 49

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)cyclobutane-1-carboxylate 527.2 50

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclo- butane-1-carboxylate 541.2 51

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2- oxoethoxy)phosphoryl)amino)cyclohexane-1-carboxylate 555.4 52

Isopropyl 1-((((((R)-1-(6-amino- 9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1- isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclo- hexane-1-carboxylate 569.4

EXAMPLE 53

Assessing Antiviral Potency in a Multiple Round HIV-1 Infection Assay(Viking Assay)

The antiviral activity of the tenofovir prodrugs of the Examples hereinwas assessed in an assay that measures the rate of replication of HIV incell culture, termed the Viking assay (Viral KINetics in Green cells)and performed as follows. HIV-1 replication was monitored usingMT4-gag-GFP clone D3 (hereafter designated MT4-GFP), which are MT-4cells modified to harbor a GFP reporter gene, the expression of which isdependent on the HIV-1 expressed proteins tat and rev. Productiveinfection of an MT4-GFP cell with HIV-1 results in GFP expressionapproximately 24 h post-infection. MT4-GFP cells were maintained at 37°C./5% CO2/90% relative humidity in RPMI 1640 supplemented with 10% fetalbovine serum, 100 μ/ml penicillin/streptomycin, and 400 μg/ml G418 tomaintain the reporter gene. For infections, MT4-GFP cells were placed inthe same medium lacking G418 and infected overnight with HIV-1 (H9/IIIBstrain) virus at an approximate multiplicity of infection of 0.01 in thesame incubation conditions. Cells were then washed and re-suspended ineither RPMI 1640 supplemented with 10% or 50% normal human serum (NFIS)at 1.6×10⁵ cells/mL (10% NHS or 50% NHS, respectively). Compound plateswere prepared by dispensing compounds dissolved in DMSO into wells of384 well poly-D -lysine-coated plates (0.2 μl/well) using an ECHOacoustic dispenser. Each compound was tested in a 10-point serial 3-folddilution (typical final concentrations: 8.4 μM-0.42 nM). Controlsincluded no inhibitor (DMSO only) and a combination of three antiviralagents (efavirenz, indinavir, an in-house integrase strand transferinhibitor at final concentrations of 4 μM each). Cells were added (50μL/well) to compound plates and the infected cells were maintained at37° C./5% CO2/90% relative humidity.

infected cells were quantified at two time points, ˜48 h and ˜72 h post-infection, by counting the number of green cells in each well using anAcumen eX3 scanner. The increase in the number of green cells over ˜24 hperiod gives the reproductive ratio, R₀, which is typically 5-15 and hasbeen shown experimentally to be in logarithmic phase (data not shown).Inhibition of R₀ is calculated for each well, and IC₅₀s determined bynon-linear 4-parameter curve fitting. Assay IC₅₀ results are shown inTable 2.

EXAMPLE 54

Prodrug Stability Assay in Bio-Relevant Media

The following assay was employed to evaluate the stability of theprodrugs in simulated gastrointestinal tract conditions. Preparation offasted state simulated intestinal fluid (FaSSIF) using Phares SIF Powderwas carried out according to protocols from Phare Drug Delivery AG(Baselland, Switzerland). For sample preparation, 10 μL stock solutions(10 mM) of prodrug substance in DMSO was added to 990 μL of 0.5 mg/mLPancreatin solution (Fisher CAS#8049-47-6) in FaSSIF. Two samples wereprepared for each compound at initial. If the sample was clear solutionat the beginning, ran one sample directly as initial by HPLC; if thesample was not clear at starting, diluted the sample by 100% ACN. Putthe other sample under 37° C. and observed the sample at 5 h time-point.At 5 h time point, if the sample was clear solution then performed HPLCanalysis directly; if it was not clear solution, diluted the sample by100% ACN and assayed by HPLC. All the samples were vortexed for 3 minand observed before injection. For the diluted samples, the area will bemultiplied by a factor when data analysis. The analysis was carried outwith an Agilent 1100 series HPLC with autosampler. The column wasusually a Poroshell 120 EC -C18, 4.6×50 mm, 2.7 μm. The flow rate was1.8 mL/min, and the injection volume was 5 or 10 μL. UV detection was inthe range 210-400 nm. The mobile phase consisted of solvent A (waterplus 10 mM tetrabutylammonium bromide) and solvent B (acetonitrile) witha gradient of 90% solvent A at 0 min changing to 95% solvent B over 6min, maintained for 1.5 min, then reverting to 90% solvent A over 1.6min. The area of the parent in prodrug at 5h time point was divided bythe area of the parent in prodrug at 0 h time point, to generate the %claimed parent ratio, which are summarized in Table 2 for GI Tractstability.

EXAMPLE 55

Pharmacokinetic Studies in Dogs—In Vivo Dog PK

Prodrugs were administered to beagle dogs through intravenous (IV) andoral (P.O.) administrations in a non-crossover manner. The IV dose wasprepared in 20% hydroxypropyl β-cyclodextrin (HPBCD) and wasadministered via cephalic or saphenous vein, The P.O. dose was preparedin 10% polysorbate 80 (Tween 80) and was administered via gavage.

Blood samples were serially collected following dose administration forup to 48 hr and plasma was separated by centrifugation. Theconcentrations of prodrugs in dog plasma were determined by LC-MS/MSassay following a protein precipitation step and addition of anappropriate internal standard (labetalol, imipramine or diclofenac).Quantification was done by determining peak area-ratios of the prodrugsand tenofovir to the internal standard. Additional blood sample(s) wascollected following dose administration for up to 24 hr. Peripheralblood mononuclear cells (PBMCs) were isolated by centrifugation, usingtubes and reagents specified for such application. The concentrations oftenofovir and/or its phosphate conjugate(s) in PBMCs were determined byan LC-VIS/MS assay following a protein precipitation step and additionof an appropriate internal standard (labetalol, imipramine ordiclofenac), Quantification was done by determining peak area-ratios oftenofovir and/or its phosphate conjugate(s) to the internal standard.

Pharmacokinetic parameters were obtained using non-compartmental methods(Watson®). The area under the plasma concentration-time curve(AUC_(0-t)) was calculated from the first time point (0 min) up to thelast time point with measurable drug concentration using the lineartrapezoidal or linear/log-linear trapezoidal rule. The IV plasmaclearance was calculated by dividing the dose by AUC_(0-inf). Theterminal half-life of elimination was determined by unweighted linearregression analysis of the log-transformed data. The time points fordetermination of half-life were selected by visual inspection of thedata. The volume of distribution at steady state (Vd_(ss)) was obtainedfrom the product of plasma clearance and mean residence time (determinedby dividing the area under the first moment curve by the area under thecurve). The maximum plasma concentration (C_(max)) and the time at whichmaximum concentration occurred (T_(max)) were obtained by inspection ofthe plasma concentration-time data. Absolute oral bioavailability (%F)was determined from dose -adjusted IV and P.O. AUC ratios of theprodrug. Table 2 shows in vivo dog PK data in the form of TFV-DPconcentrations (μM) in dog PBMCs at 24 h following a 10 mg/kg P.O. doseof the indicated prodrug

TABLE 2 Viking, IC₅₀ Viking, IC₅₀ GI Tract In Vivo (10% NHS) (50% NHS)stability Dog Example (nM) (nM) (%) PK (μM)  1A 14.69 61.08 85.74  1B7.43 12.08 99.82  2A 4.05 22.15 61.61  2B 18.22 70.86 91.95 22.11  3A36.62 121.1 96.63  3B 29.10 47.88 95.71  4A 13.88 86.55 99.07 21.59  4B19.42 157.50 98.87  5A 5.71 45.00  5B 5.73 23.42  6A 3.14 15.20 95.97 6B 2.14 7.99 73.16  7A 5.03 21.02 15.36  7B 0.37 1.01 76.64 11.75  8A6.76 27.02 94.11  8B 22.68 161.2 71.6  9A 4.20 26.06  9B 1.00 2.78 6.1610A 26.29 142.00 10B 85.63 356.90 11A 24.84 66.17 11B 3.98 22.80 12A5.71 25.18 98.95 12B 6.07 13.49 13A 6.17 51.07 70.30 13B 9.44 60.2099.72 14A 10.01 23.53 14B 5.40 36.19 15A 9.98 28.51 15B 10.31 44.17 16A2.61 7.50 16B 4.75 17.05 17A 6.39 12.72 17B 6.70 18.35 99.32 18A 6.5916.46 76.31 18B 2.69 8.77 91.31 23.23 19A 14.01 49.69 19B 3.01 9.93100.00 20 20.97 96.96 7.19 21 6.15 11.99 99.36 22 4.65 17.67 79.24 232.67 8.19 4.28 24 4.78 27.80 98.30 25 2.44 10.5 79.61 26 3.10 8.74 9.0127 10.08 13.31 28 6.38 19.71 29 9.22 17.21 30 4.38 11.98 31 2.39 7.5785.48 32 3.34 23.18 33 1.69 5.06 26.96 34 3.57 12.92 35 3.83 14.00 362.39 6.04 88.87 37 10.28 63.08 38 2.68 6.26 96.64 39 3.43 12.63 40 19.1758.71 41 2.87 12.49 58.50 42 5.00 21.77 73.54 43 6.15 20.37 44 1.42 1.8145 15.61 62.58 99.44 46 385.80 702.80 98.18 47 91.61 209.20 93.55 4813.29 37.31 49 19.55 36.99 100.50 50 8.66 34.30 51 9.80 24.86 99.70 5210.83 45.60

What is claimed is:
 1. A compound of structural Formula I:

or a pharmaceutically acceptable salt thereof, wherein: R¹ and R² are each independently selected from (a) H, (b) —C₁₋₄alkyl, (c) —C₁₋₄alkyl substituted with —OH, —SH, —SCH₃, —NH₂ or —NH—C(═NH)—NH₂, (d) —CH₂-phenyl, (e) —CH₂-phenol, (f) —(CH₂)₁₋₂—COOH, (g) —(CH₂)₁₋₂—CONH₂, (h) —CH₂-1H-indole, (i) —CH₂-imidazole, (j) aryl or (k) heteroaryl; R³ is selected from: (a) —C₁₋₁₀alkyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(7a), —SH, —NR^(9a)R^(10a), —C₃₋₆cycloalkyl or spiro—C₃₋₆cycloalkyl, (b) —CH₂-phenyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (c) —C₃₋₈cycloalkyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (d) aryl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (e) —C₁₋₅alkyl—X—C₁₋₅alkyl wherein X is O, S or NH, (f) heteroaryl unsubstituted or substituted with one to three substituents independently selected from fluoro chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, or (g) a heterocyclic ring unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl; R⁴ is selected from: (a) —C₁₋₁₀alkyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(7b), —SH, —NR^(9b)R^(10b), —C₃₋₆cycloalkyl or spiro—C₃₋₆cycloalkyl, (b) —CH₂-phenyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (c) —C₃₋₈cycloalkyl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (d) aryl unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (e) —C₁₋₅alkyl—X—C₁₋₅alkyl wherein X is O, S or NH, (f) heteroaryl unsubstituted or substituted with one to three substituents independently selected from fluoro chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, or (g) a heterocyclic ring unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl; R⁵ and R⁶ are each independently selected from (a) —C₁₋₄alkyl, (b) —C₁₋₄alkyl substituted with —OH, —SH, —SCH₃, —NH₂, —NcyC(═NH)—NH₂, (c) —CH₂-phenyl, (d) —CH₂-phenol, (e) —(CH₂)₁₋₂—COOH, (f) —(CH₂)₁₋₂—CONH₂, (g) —CH₂-1H-indole, (h) —CH₂-imidazole, (i) aryl or (j) heteroaryl; or R⁵ and R⁶ are joined together with the carbon to which they are both attached to form —C₃₋₆cycloalkyl or a 4 to 6-membered heterocyclic ring; R^(7a) and R^(7b) are each independently selected from —H or —C₃₋₆cycloalkyl; R^(8a) and R^(8b) are each independently selected from —H, —C₁₋₃alkyl or —C₃₋₆cycloalkyl; R^(9a) and R^(10a) are each independently selected from —H, —C₁₋₃alkyl or —C₃₋₆cycloalkyl; and R^(9b) and R^(10b) are each independently selected from —H, —C₁₋₃alkyl or —C₃₋₆cycloalkyl.
 2. The compound of claim 1 or a pharmaceutically acceptable salt thereof wherein one of R¹ is —H or —C₁₋₄alkyl, and R² is —H, —C₁₋₄alkyl or —CH₂-phenyl.
 3. The compound of claim 2 or a pharmaceutically acceptable salt thereof wherein R⁵ and R⁶ are independently selected from —C₁₋₄alkyl, or R⁵ and R⁶ are joined together with the carbon to which they are both attached to form —C₃₋₆cycloalkyl.
 4. The compound of claim 3 or a pharmaceutically acceptable salt thereof wherein R³ is: (a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH, —CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂, (b) —CH₂-phenyl, unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (d) phenyl or naphthyl, each unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, (e) —CH₂CH₂OCH₃, —CH₂CH₂CH₂OCH₃, —CH₂CH₂SCH₃, —CH₂CH₂CH₂SCH₃, —CH₂CH₂NHCH₃, —CH₂CH₂CH₂NHCH₃, (f) pyridyl, unsubstituted or substituted with one to three substituents independently selected from fluoro chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl, or (g) piperidinyl, pyrrolidinyl, tetrahydrofuranyl, or tetrahydropyranyl, each unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8a), —SH, —NR^(9a)R^(10a) or —C₁₋₃alkyl.
 5. The compound of claim 4 or a pharmaceutically acceptable salt thereof wherein R⁴ is: (a) —C₁₋₈alkyl, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH₂SH, —CH₂CH₂CH₂SH, —CH₂CH₂NH₂, —CH₂CH₂CH₂NH₂, (b) —CH₂-phenyl, unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (c) —C₃₋₆cycloalkyl, unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (d) phenyl or naphthyl, each unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, (e) —CH₂CH₂OCH₃, —CH₂CH₂CH₂OCH₃, —CH₂CH₂SCH₃, —CH₂CH₂CH₂SCH₃, —CH₂CH₂NHCH₃, —CH₂CH₂CH₂NHCH₃, (f) pyridyl, unsubstituted or substituted with one to three substituents independently selected from fluoro chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl, or (g) piperidinyl, pyrrolidinyl, tetrahydrofuranyl, or tetrahydropyranyl, each unsubstituted or substituted with one to three substituents independently selected from fluoro, chloro, bromo, —OR^(8b), —SH, —NR^(9b)R^(10b) or —C₁₋₃alkyl.
 6. The compound of claim 1 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each independently unsubstituted or substituted and each is independently selected from —C₁₋₈alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH2-phenyl.
 7. The compound of claim 2 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each independently unsubstituted or substituted and each is independently selected from —C₁₋₈alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH₂-phenyl.
 8. The compound of claim 3 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each independently unsubstituted or substituted and each is independently selected from —C₁₋₈alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH2-phenyl.
 9. The compound of claim 1 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each unsubstituted and independently selected from —C₂₋₆alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH₂-phenyl.
 10. The compound of claim 2 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each unsubstituted and independently selected from —C₂₋₆alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH₂-phenyl.
 11. The compound of claim 3 or a pharmaceutically acceptable salt thereof wherein R³ and R⁴ are each unsubstituted and independently selected from —C₂₋₆alkyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, or —CH₂-phenyl.
 12. The compound of claim 1 or a pharmaceutically acceptable salt thereof, wherein: R¹ is —H or —CH_(3;) R² is —H, —CH₃ or —CH₂-phenyl; R³ is selected from —C₁₋₈alkyl, —C₃₋₆cycloalkyl or —CH₂-phenyl; R⁴ is selected from —C₁₋₈alkyl or —C₃₋₆cycloalkyl; and R⁵ and R6 are both —CH₃; or R⁵ and R⁶ are joined together with the carbon to which they are both attached to form —C₃₋₆cycloalkyl.
 13. The compound of claim 1 that is: Isopropyl 2-4(S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-y0oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-ypoxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-oxo-2-(pentyloxy)ethoxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-oxo-2-(pentyloxy)ethoxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(4R)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-oxo-1-(pentyloxy)propan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-oxo-1-(pentyloxy)propan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Butyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy) -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Butyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy) -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclohexyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclohexyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclohexyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclohexyl 2-(((R)-((((R)1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclobutyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclobutyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclobutyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclobutyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((S)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-(((R)-((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl) (((R)-1 -is opr op oxy - 1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Propyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Propyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Propyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1-i sopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Butyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Butyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Butyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1 -isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Ethyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Ethyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Ethyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((R)-1-i sopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isobutyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-ypoxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Isobutyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxo-3-phenylpropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxo-3-phenylpropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy)-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-(cyclopentyloxy)-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Pentyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-(cyclopentyloxy)-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Isobutyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-(cyclopentyloxy) -1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Propyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-(cyclopentyloxy)-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Cyclopentyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-ethoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Benzyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Benzyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-ethoxy-1-oxopropan -2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Hexyl 2-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)((1-isopropoxy-2-methyl-1-oxopropan-2-yl)oxy)phosphoryl)amino)-2-methylpropanoate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)cyclopentane-1-carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)cyclopropane-1-carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclopropane-1 -carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclopentane-1-carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)cyclobutane-1-carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclobutane-l-carboxylate; Isopropyl 1-((((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(2-isopropoxy-2-oxoethoxy)phosphoryl)amino)cyclohexane-1-carboxylate; or Isopropyl 1-((((((R) -1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)(((S)-1-isopropoxy-1-oxopropan-2-yl)oxy)phosphoryl)amino)cyclohexane-1-carboxylate; or a pharmaceutically acceptable salt thereof.
 14. A pharmaceutical composition comprising an effective amount of the compound of claim 1, or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier.
 15. The pharmaceutical composition of claim 14 further comprising an effective amount of one or more additional HIV antiviral agent selected from HIV protease inhibitors, HIV integrase inhibitors, non-nucleoside HIV reverse transcriptase inhibitors, nucleoside HIV reverse transcriptase inhibitors, HIV fusion inhibitors and HIV entry inhibitors.
 16. A method for the prophylaxis or treatment of infection by HIV or for the prophylaxis, treatment, or delay in the onset of AIDS in a subject in need thereof which comprises administering to the subject an effective amount of the compound of claim 1 or a pharmaceutically acceptable salt thereof.
 17. The method of claim 16 further comprising administering to the subject an effective amount of one or more additional HIV antiviral agent selected from HIV protease inhibitors, HIV integrase inhibitors, non-nucleoside HIV reverse transcriptase inhibitors, nucleoside HIV reverse transcriptase inhibitors, HIV fusion inhibitors and HIV entry inhibitors.
 18. The pharmaceutical composition of claim 14 further comprising an effective amount of one or more additional HIV antiviral agent selected from: abacavir, abacavir + lamivudine, abacavir + lamivudine + zidovudine, amprenavir, atazanavir, AZT, capravirine, darunavir, ddC, ddI, ddI (enteric coated), delavirdine, dolutegravir, doravirine, efavirenz, efavirenz + emtricitabine + tenofovir DF, 4′-ethynyl-2-fluoro-2′-deoxyadenosine, elvitegravir, emtricitabine, emtricitabine + tenofovir DF, emivirine, enfuvirtide, enteric coated didanosine, etravirine, fosamprenavir calcium, indinavir, lamivudine, lamivudine + zidovudine, lopinavir, lopinavir + ritonavir, maraviroc, nelfinavir, nevirapine, PPL-100, raltegravir, rilpivirine ritonavir, saquinavir, stavudine, tipranavir, or vicriviroc. 